Hidden gems is a hard question to answer because if the game is really a gem, then lots of people know about it, thus not hidden. Similarly, games that may not appeal to many, may appeal to you (for example, I really, really like Too Human, and those that liked the game, love the game, but many people hate the game as well). Thus there are many games that fall in that catagory. To me the key is pricing. Many games are not worth $60, but at $10-15, they are really good. There are so many games (like Darkness noted above), that may not appeal to everyone, but for $10, almost become a no-brainer. I think that any game that gets a metacritic above 75% is a no brainer blind buy when you can get it for $10.
Xbox and PS3 Gamertag = Xerostomia
Wii = I'm too lazy to get the number