TNA Wrestling, Game Dev Story (maybe not hidden anymore), Imprisoned, Team Alpha, and of course Strongholds. Orions is good too, but such a battery drainer!
Xenome It's an open world, Fallout-style RPG that NO ONE talks about... The graphics and controls are a bit clunky, but it's not that bad overall.
I'm actually not a fan of casual games... But I went to download Pix N Love Rush (for iPad) - it looks fun. And I saw on the sidebar of iTunes, other games by the developer... So, not only did I get Pix N Love Rush for 2 bucks... But! I also walked away with the game I now consider a hidden gem: "Snake Galaxy" for iPad, it was on sale for a low .99 cents! It's got beautiful graphics, I mean insanely beautiful graphics. I had wanted it for months, but not at 5 dollars. I finally ended up though jumping in. It's pretty good, definite buy for .99 cents. I'm not plugging the dev or anything, but I consider the game a hidden gem and a steal at that price. Could use more levels though.
I would say my hidden gem is THE HERO. Its a great game and its published by Chillingo but no one talks about it.
It once was, but now, there are several better games coming out, and hype threads ended up consuming it. I never found it to be my kind of game, so i passed.