I decided to post this thread after thinking about the insane number of awesome, little known MAME games. Please post any MAME recommendations you have here. Here's mine:

Action/Fighting:
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Daraku Tenshi - The Fallen Angels
Metamoqester
Knights of Valour/Knights of Valour Plus
Mobile Suit Gundam EX Review
Power Instinct 3 - Groove on Fight
Dead or Alive ++
Spiderman: The Video Game
Asura Blade - Sword of Dynasty
Batman
Dragonball Z & Dragonball Z 2
(Note: I'm not going to list the Mortal Kombats, Double Dragons, Final Fight, Street Fighter, ect. here because everyone knows they rock.)

Shooters
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ESP Ra. De.
Armored Police Batrider
Guwange
Gunbird
Gunbird 2
Space Bomber
Battle Garegga
Battle Barkraid & Battle Barkraid Unlimited Version
Giga Wing
Mars Matrix
DonPachi
DoDonPachi
Dangun Feveron
Gradius 3
R-Type Leo
Ultra X Weapons
Dimahoo
Cyvern
Dragon Blaze

Sports
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Super Slams (Based on the anime Slam Dunk. It's got an animated intro!)
Cool Boarders Arcade Jam
Baseball Stars 2
NBA Jam T.E.
NBA Hangtime
Golden Tee Golf II

Lightgun
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Alien 3: The Gun
Terminator 2: The Arcade Game
Zero Point 2
Chiller

Puzzle
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Pnickies
The Next Tetris
Tetris Plus 2
Puyo Puyo
Puyo Puyo Sun

Please feel free to post your recommendations in this thread too. These aren't all the recommendations I have, I'm just lazy at the moment. I'll update it later. Oh and you can get all these from http://www.emuconnect.com and http://www.romnation.net (Be sure to use Firefox for EmuConnect)

Enjoy!