Here's some sleepers...
Jaki Crush- The follow up to Devils Crash,this pinball sleeper offers some very cool bonus rounds and cool tunes. Has a darker, more sinister Asian vibe to it than Devils Crash.
Hard as hell to find, but worth scoring.
Tetris Battle Gaiden- Easily my fave Tetris variant of all time. This game has selectable characters, who all have their own animated spells that do horrible things to their opponent's board. From the bombs that blow holes in your opponents wall, to a fax attack that faxes whatever your playfield looks like to you opponent, this game is blazing original, and has some really cool tunes to boot. One of the sweetest sleepers I've ever encountered.