As an indie gamer, I've got a pretty good library of obscure games... let's just list off some of my favorites.
Advent Rising is an EPIC and extremely fun game for PC and the original Xbox; not the PS2 to my knowledge. The storyline was written by none other than Orson-Scott Card himself, and though there are some minor graphical hiccups, it's overall an underrated gem. You can grab it for ten bucks from Good Old Games.
Freelancer is an older space-flight sim for the computer, and is something like a singleplayer version of EVE online with a massive, dynamic universe, an amazing story, and lots of stuff to find and ships to buy and outfit.
Guns of Icarus is an indie title available on Steam where you play a lone airshipman trying to keep his ship intact, his cargo secure, and his life present by fending off hordes of pirates and repairing his vessel on the fly. The freedom to pick your own route at varying levels of difficulty means that the single-player can hold out for a while, but the game is most fun in Multiplayer, with a number of players working on defending and repairing one ship.
Impossible Creatures is a 2002 Real Time Strategy, and in most aspects, is not unlike other contenders in the genre, but is most unique in the area of its units. You are given a large selection of animals to pick from, and you can combine two of said animals in a ridiculous number of unique ways, giving you the freedom to unleash a completely custom-built army.
Painkiller is pretty well known, but not well-known enough.
The Penumbra trilogy is a series of physics-based psychological horror games from developer Frictional, probably most well known for their recent title, Amnesia. They're not all that different from Amnesia in terms of basic premise and game-play, but are terrifying in a whole different league, forcing you to contend with mutated wolves and the victims of a horrendous parasitic infestation, often with no form of defense other than your wit and your sprint key.
Second Sight is an awesome action game where you play a man gifted with extremely powerful psionic abilities as he wakes up and struggles to recall his past... though he finds that events are changing as he remembers them. A deep and complex story combined with frantic telepathic combat make this an underrated gem.
The Void is a completely unique game where you struggle to survive in the realm between life and death, surviving on nothing other than the colour sparsely scattered through this dark, almost black and white realm, and forced to give of your own life energy to combat the threats lingering there. A warning, though: it is EXTREMELY difficult, even early on.
Chrome is a fun shooter that has been described as the working man's Crysis, a low-draw action game where you have the tactical option of various cybernetic enhancements to your body.
Chaser is another shooter that has existed in almost complete obscurity for a while now, and while it is remarkably dated in terms of graphical performance, its story is unexpectedly good, and if you can look past the graphics and flawed gameplay, it can be extremely enjoyable.
Tank Universal is a cross between Tron and Battlezone; a tank sim/RTS indie mashup that takes place within the computerized world of a virtual helmet. The campaign's difficulty spikes at one point, and can be extremely unforgiving as it goes on, and the story is somewhat lackluster, but the sheer variety in gameplay more than makes up for it.
Uplink is a SERIOUSLY kickass hacking simulator; never have I felt more powerful. It's cheap, it's fun, and any computer will run it. Why not?
...
I feel like I'm forgetting something.
...
RIGHT!
Minecraft ^-^