Wow, I saw a lot of ones listed that few people I know have even heard of. Guardian's Crusade, Threads of Fate, Azure Dreams... I might have missed a couple though, so if these were mentioned already or aren't considered sleepers, I apologize.
Battle Hunter (PSX) - An odd little game. It's not much to look at, especially when it comes to the "dungeons", but it might be worth a look if you like TBTs. I think it's one of those love it or hate it games. At least of the few I talked to that actually played it, I'm the only person that can find a middle ground to the game.
Top Shop (PSX) - From the same bunch that did Battle Hunter. It's more a board-game type of game... but I just wanted to add it in the one-in-a-million chance someone here might have heard of it. Nobody else I talked to has.
Evolution (Dreamcast) - NOT the Gamecube version. I have that... it may have voice acting unlike the DC version, and combine both 1 and 2, but what they cut out in exchange is overwhelming. It's basically a dungeon-crawler, so if you know what that is, you have an idea of what to expect in terms of gameplay. Still, I thought the story was pretty good from what I saw of it... a bit predictable in some spots though.
Rhapsody: A Musical Adventure (PSX) - Is this a sleeper or a singer? Either way... if you've ever heard of Disgaea, Phantom Brave or Nippon Ichi, you'll have an idea about what this game is like. It has plenty of problems... the dungeons are about as good as the ones in Battle Hunter, and the battles are... well... pathetic. Still, it's pretty original in the fact that characters just start singing throughout the game... hence the title.
I could have sworn I had more to list... darn memory. Oh well.
Gaming World Forums
Sleeper Rpgs