Now I've played a game, that as far as I know, is the only game that has 'sold out' on Steam. Others have been taken down, mostly due to dubious quality and to EA being a dick, but this is a special case. OK, wow, I had no clue that the list was that long.
Prey puts you in the shoes of a Cherokee man named Tommy. The game starts off decent enough in his girlfriend's bar, when you punch some guys that get frisky, then all hell breaks loose. And by 'all hell', I mean 'an alien spaceship', and by 'breaks loose', I mean 'abducts everyone'. Someone sets them up the bomb, and you escape, but your girlfriend is taken deeper into the ship, and grandpa dies. Eventually, you discover that this ship is freaking huge, and the aliens are there to harvest all living things and process them into some sort of organic sludge. There are messages on screens telling the Hunters to not play with the protein. Almost everything in the ship is alive, damp, and squishy, even your guns. For instance, small tripodal creatures scurry around the floor, and when you pop their legs off, they become live grenades. Grandpa's ghost comes around from time to time and teaches old Cherokee warrior things.
I like some of the level elements that Prey used. I specifically enjoyed the areas that disregard gravity. It really messes with you, because you have to keep in mind which way is down when you look somewhere else. Wall walking makes for some interesting level design, and you can easily meet someone upside-down. I also really liked the asteroids that were scattered around. These things would have very low gravity, and you can feel yourself being curved around the thing when walking on it. Yes, I enjoyed jumping around on those things quite a bit. Oh, and there are portals. Prey was doing portal years before Portal did portals. It wasn't uncommon to be running down a hallway and see a portal pop open and a hunter come out of it. Empty boxes sometimes lie around, only to have a whole side turn into a portal.
This game was also the soft-of sister game to Duke Nukem Forever. Both were announced by the same company at nearly the same time, but somehow this was probably the better of the two. And by 'probably', I mean 'I only played this one'. Both also had rather totured development processes, lasting over 10 years. But that's not the end of the story. It was the early days of digital game distribution, and the store that was tagged to sell Prey, Triton, shut down a few months after release. Steam picked it up. During Steam's 2009 Christmas sale, Prey was discounted so much that it sold out.
Prey 2 looked pretty good, from what material was released for it. It seemed like a cross between Mass Effect and Mirror's Edge, and was only vestigially connected to the original. Instead of being a Cherokee warrior on an epic sized alien ship, you would be a former Air Marshall wandering around an alien city on the dark side of some world. From what I heard, you and Tommy (he is still around for some reason) are the only humans on this world. The gameplay would consist of taking bounties on bad guys, then running them down in the streets. Like its predecessor, Prey 2 already has a tumoutous development process, from being on, to being sold and bought, to there being a labor dispute, to, apparently, back on track. I'm not sure if this will make it or not.
If you're into interesting ideas that not many games have done, try it. Good luck finding it though. I'm not sure if these things (like messing with gravity) are technically difficult to do, confusing, bad, unpopular, or not exposed enough for other games to use.