It really depends on what genres you like. Everything tiktektak posted is solid, as well as the following:
3D Dot Game Heroes - If you like 2D Zelda games, you'll like this.
Binary Domain - Underrated TPS.
Dragon's Crown - 2D Brawler RPG.
Katamari Forever - Probably the best Katamari game since the PS2 ones.
Persona 4: Arena - Solid anime-styled 2D fighting game.
Sengoku Basara: Samurai Heroes - Dynasty Warriors, but with Devil May Cry combat and more personality.
Spec Ops: The Line - Underrated TPS with an excellent story.
Tales of Symphonia Chronicles - Re-release of one of the best JRPGs on the Gamecube.
Valkyria Chronicles - Not really unknown, but an excellent game.
Vanquish - The best TPS on the console, hands down.
Way of the Samurai 3/4 - Essentially a chose-your-own-adventure book. 34 endings and a great combat system.
Yakuza 3/4 - A modern JRPG with 3D brawling instead of traditional combat.
I'm not sure what your requirements for "unknown" are, but this should get you started. There are plenty of weird Japanese titles on the PS3, so there are a lot of hidden gems.
(As a side note: If you've never played an Earth Defense Force game, buy EDF 2025 and fix that)
