Impressive T6W28 Music (Neo Geo Pocket & Neo Geo Pocket Color)
A PSG chip which is very similar to a 2x SN76489 setup: 6 (?) square wave tone generators and 2 (?) noise generators with stereo capability + direct access to the two 6-bit digital-to-analog converters. Click the image for a link to a page with more info.
The wikipedia info on the chip is partially incorrect, the noise channel can play in stereo (and the panning is more advanced than just left/mid/right hard panning - see for example Strategy, Path of the Fighter or Fatal Flaw from SNK vs. Capcom: Match of the Millennium).
I'm not sure why, but the second chip's pulse channels seem to have been very rarely used for music beyond the noise channel+1 silent pulse channel for drums (aka free range noise mode on SMS where the noise channel uses the pitch of one pulse channel for a wider range of sounds). Even if sfx is played on the second chip, you'd have 1 pulse channel free to add more harmonies or echo effects. The only examples I've found so far are Options, Got Emerald and Game Over from Sonic the Hedgehog: Pocket Adventure. Given how it's used there, maybe there's just one pulse channel for the second chip? The noise channel for the first chip seems to not have been used either.
Another thing that was barely used is periodic noise mode, which sounds similar to a saw wave and could be used for bass - one rare example of it being used can be found in Game Over from Sonic the Hedgehog: Pocket Adventure.
The 6-bit DACs were never used in conjunction with the "SMS" parts of the chip as far as I'm aware, but a couple of games that used them for music are Faselei! (streamed vocal intro track) and Ganbare Neo Poke-Kun (short jingles independent of the PSG music).
The only other things of note that I could find are a raspy noise-like effect used on the pulse channels in certain tracks (reminiscent of certain Spectrum OSTs; see for example Activation, Tyranny, Gouki's Theme, Annihilation or Dan's theme from SNK vs. Capcom: Match of the Millennium or Wily Capsule from Rockman Battle & Fighters). There's also some nice pulse channel percussion in Geisha!! Final Match from Big Bang Pro Wrestling and in First Contact from Metal Slug: 2nd Mission.
King of Fighters R-2 (NGPC), Intro, Alternate
Sonic the Hedgehog: Pocket Adventure (NGPC)
SNK vs. Capcom: Match of the Millennium (NGPC)
King of Fighters R-1 (NGP)
Fatal Fury: First Contact (NGPC)
Big Bang Pro Wrestling (NGPC)
Rockman Battle & Fighters (NGPC)
Cotton: Fantastic Night Dreams (NGPC)
Faselei! (NGPC)(streaming DAC, vocals)
SNK vs. Capcom: Card Fighter's Clash - Capcom Version & SNK Version (NGPC)(shared tracks)
The King of Fighters: Battle de Paradise (NGPC)(in-game)
Metal Slug 2nd Mission (NGPC)
SNK Gals' Fighters (NGPC)(in-game)
Neo Geo Cup '98 & Neo Geo Cup '98 Plus Color (NGP/NGPC)(in-game)
Renketsu Puzzle Tsunagete Pon! (NGP)(in-game)
Puzzle Link 2 (NGPC)(in-game)
SNK vs. Capcom: Card Fighter's Clash 2 Expand Edition (NGPC)(in-game)
Puzzle Link (NGPC)(in-game)
Dark Arms: Beast Buster (NGPC)(in-game)
Samurai Shodown! (NGP)(in-game), Alternate
Biomotor Unitron (NGPC)(in-game)
Memories Off Pure (NGPC)(in-game)
Metal Slug 1st Mission (NGPC)
Neo Derby Championship (NGPC)(in-game)
Infinity Cure (NGPC)(in-game)
Bust-a-Move Pocket/Puzzle Bobble Mini (NGPC)
Biomotor Unitron 2/Kikou Seiki Unitron (NGPC)(in-game)
Magical Drop Pocket (NGPC)(in-game)
Pocket Reversi (NGPC)(in-game)
Crush Roller (NGPC)(in-game)
Ganbare Neo Poke-Kun (NGPC)(in-game)
Bakumatsu Roman Dainimaku: Gekka no Kenshi - Tsuki ni Saku Hana/The Last Blade 2 (NGPC)(in-game)
Nige-ron-pa (NGPC)(in-game)
Samurai Shodown! 2 (NGPC)
Delta Warp (NGPC)
Pocket Tennis & Pocket Tennis Color (NGP/NGPC)(in-game)
Legend of Ogre Battle Gaiden: Prince of Zenobia (NGPC)
Cool Cool Jam (NGPC)(in-game)
Cool Boarders Pocket (NGPC)(in-game)
Neo 21 (NGPC)(in-game)
Neo Baccarat (NGPC)(shares tracks with Neo Cherry Master and Neo 21), Koi Koi Mahjong (NGPC), Evolution: Eternal Dungeons (NGPC), Bikkuriman 2000 Viva! Pocket Festival (NGPC), Neo Turf Masters (NGPC), Mizuki Shigeru Youkai Shashinkan (NGPC), Neo Cherry Master & Neo Cherry Master Color (NGP/NGPC), Neo Dragon's Wild (NGPC)(shares some tracks with Neo Cherry Master), Neo Mystery Bonus (NGPC)(shares some tracks with Neo Cherry Master), Super Real Mahjong Premium Collection (NGPC) - find hq,
Dive Alert: Becky's Version & Dive Alert: Matt's Version (NGPC), Pocket Love: If (NGPC), Doko Demo Mahjong (NGP), Puyo Pop (NGPC), Neo Geo Pocket BIOS, PachiSlot Aruze Kingdom - e-CUP (NGPC), Melon Chan's Growth Diary (NGP), PachiSlot Aruze Kingdom - Dekahel (NGPC), PachiSlot Aruze Kingdom - Ohanabi (NGPC), PachiSlot Aruze Kingdom - Azteca (NGPC), PachiSlot Aruze Kingdom - Porcano 2 (NGPC), PachiSlot Aruze Kingdom - Del Sol 2 (NGPC), Pachinko Hissho Guide: Pocket Parlor (NGPC), Shanghai Mini (NGPC), Picture Puzzle (NGPC), Neo Poke Pro Yakyuu (NGPC), Baseball Stars & Baseball Stars Color (NGP/NGPC), PachiSlot Aruze Kingdom: Hanabi (NGPC), PachiSlot Aruze Kingdom - Ward of Lights (NGPC), Mezase! Kanji-ou (NPGC), Dynamite Slugger (NGPC), Densha de Go! 2 (NGPC), Shogi no Tatsujin: Master of Syougi & Shogi no Tatsujin: Master of Syougi Color (NGP/NGPC), Party Mail (NGPC), Pac-Man (NGPC),
Find: Soreyuke! Hanafuda Dojo
The wikipedia info on the chip is partially incorrect, the noise channel can play in stereo (and the panning is more advanced than just left/mid/right hard panning - see for example Strategy, Path of the Fighter or Fatal Flaw from SNK vs. Capcom: Match of the Millennium).
I'm not sure why, but the second chip's pulse channels seem to have been very rarely used for music beyond the noise channel+1 silent pulse channel for drums (aka free range noise mode on SMS where the noise channel uses the pitch of one pulse channel for a wider range of sounds). Even if sfx is played on the second chip, you'd have 1 pulse channel free to add more harmonies or echo effects. The only examples I've found so far are Options, Got Emerald and Game Over from Sonic the Hedgehog: Pocket Adventure. Given how it's used there, maybe there's just one pulse channel for the second chip? The noise channel for the first chip seems to not have been used either.
Another thing that was barely used is periodic noise mode, which sounds similar to a saw wave and could be used for bass - one rare example of it being used can be found in Game Over from Sonic the Hedgehog: Pocket Adventure.
The 6-bit DACs were never used in conjunction with the "SMS" parts of the chip as far as I'm aware, but a couple of games that used them for music are Faselei! (streamed vocal intro track) and Ganbare Neo Poke-Kun (short jingles independent of the PSG music).
The only other things of note that I could find are a raspy noise-like effect used on the pulse channels in certain tracks (reminiscent of certain Spectrum OSTs; see for example Activation, Tyranny, Gouki's Theme, Annihilation or Dan's theme from SNK vs. Capcom: Match of the Millennium or Wily Capsule from Rockman Battle & Fighters). There's also some nice pulse channel percussion in Geisha!! Final Match from Big Bang Pro Wrestling and in First Contact from Metal Slug: 2nd Mission.
King of Fighters R-2 (NGPC), Intro, Alternate
Sonic the Hedgehog: Pocket Adventure (NGPC)
SNK vs. Capcom: Match of the Millennium (NGPC)
King of Fighters R-1 (NGP)
Fatal Fury: First Contact (NGPC)
Big Bang Pro Wrestling (NGPC)
Rockman Battle & Fighters (NGPC)
Cotton: Fantastic Night Dreams (NGPC)
Faselei! (NGPC)(streaming DAC, vocals)
SNK vs. Capcom: Card Fighter's Clash - Capcom Version & SNK Version (NGPC)(shared tracks)
The King of Fighters: Battle de Paradise (NGPC)(in-game)
Metal Slug 2nd Mission (NGPC)
SNK Gals' Fighters (NGPC)(in-game)
Neo Geo Cup '98 & Neo Geo Cup '98 Plus Color (NGP/NGPC)(in-game)
Renketsu Puzzle Tsunagete Pon! (NGP)(in-game)
Puzzle Link 2 (NGPC)(in-game)
SNK vs. Capcom: Card Fighter's Clash 2 Expand Edition (NGPC)(in-game)
Puzzle Link (NGPC)(in-game)
Dark Arms: Beast Buster (NGPC)(in-game)
Samurai Shodown! (NGP)(in-game), Alternate
Biomotor Unitron (NGPC)(in-game)
Memories Off Pure (NGPC)(in-game)
Metal Slug 1st Mission (NGPC)
Neo Derby Championship (NGPC)(in-game)
Infinity Cure (NGPC)(in-game)
Bust-a-Move Pocket/Puzzle Bobble Mini (NGPC)
Biomotor Unitron 2/Kikou Seiki Unitron (NGPC)(in-game)
Magical Drop Pocket (NGPC)(in-game)
Pocket Reversi (NGPC)(in-game)
Crush Roller (NGPC)(in-game)
Ganbare Neo Poke-Kun (NGPC)(in-game)
Bakumatsu Roman Dainimaku: Gekka no Kenshi - Tsuki ni Saku Hana/The Last Blade 2 (NGPC)(in-game)
Nige-ron-pa (NGPC)(in-game)
Samurai Shodown! 2 (NGPC)
Delta Warp (NGPC)
Pocket Tennis & Pocket Tennis Color (NGP/NGPC)(in-game)
Legend of Ogre Battle Gaiden: Prince of Zenobia (NGPC)
Cool Cool Jam (NGPC)(in-game)
Cool Boarders Pocket (NGPC)(in-game)
Neo 21 (NGPC)(in-game)
Neo Baccarat (NGPC)(shares tracks with Neo Cherry Master and Neo 21), Koi Koi Mahjong (NGPC), Evolution: Eternal Dungeons (NGPC), Bikkuriman 2000 Viva! Pocket Festival (NGPC), Neo Turf Masters (NGPC), Mizuki Shigeru Youkai Shashinkan (NGPC), Neo Cherry Master & Neo Cherry Master Color (NGP/NGPC), Neo Dragon's Wild (NGPC)(shares some tracks with Neo Cherry Master), Neo Mystery Bonus (NGPC)(shares some tracks with Neo Cherry Master), Super Real Mahjong Premium Collection (NGPC) - find hq,
Dive Alert: Becky's Version & Dive Alert: Matt's Version (NGPC), Pocket Love: If (NGPC), Doko Demo Mahjong (NGP), Puyo Pop (NGPC), Neo Geo Pocket BIOS, PachiSlot Aruze Kingdom - e-CUP (NGPC), Melon Chan's Growth Diary (NGP), PachiSlot Aruze Kingdom - Dekahel (NGPC), PachiSlot Aruze Kingdom - Ohanabi (NGPC), PachiSlot Aruze Kingdom - Azteca (NGPC), PachiSlot Aruze Kingdom - Porcano 2 (NGPC), PachiSlot Aruze Kingdom - Del Sol 2 (NGPC), Pachinko Hissho Guide: Pocket Parlor (NGPC), Shanghai Mini (NGPC), Picture Puzzle (NGPC), Neo Poke Pro Yakyuu (NGPC), Baseball Stars & Baseball Stars Color (NGP/NGPC), PachiSlot Aruze Kingdom: Hanabi (NGPC), PachiSlot Aruze Kingdom - Ward of Lights (NGPC), Mezase! Kanji-ou (NPGC), Dynamite Slugger (NGPC), Densha de Go! 2 (NGPC), Shogi no Tatsujin: Master of Syougi & Shogi no Tatsujin: Master of Syougi Color (NGP/NGPC), Party Mail (NGPC), Pac-Man (NGPC),
Find: Soreyuke! Hanafuda Dojo