http://teleglitch.com/VideoSurprised this hasn't yet been mentioned.
Teleglitch is a top-down, low graphics shooter with roguelike elements. Random level generation, enemy and resource placement.
The gist is you're the last surviving researcher on a planet run by a large military research corporation. Fields of study include: weapons research, tissue reanimation, mutagenic experiments and long-range teleportation.
And of course everything went horribly wrong.
There's a limited amount of crafting in the game, allowing you to modify weapons or create new ones, in addition to very useful equipment and upgrades.
The game can be very difficult at times, both due to the RNG and the limited amount of ammo available. You have to tread carefully and make every shot count if you can. There are only 9 levels, but they get progressively longer and more dangerous the deeper you go, until each level can take about 30 minutes to complete. You can only restart on a select number of levels AFTER you've beaten them. On my 3rd try I made it level 6/9, so it's not the hardest game in the world but it definitely puts up a good challenge. Especially when you start running into some of the end-game bosses.
Atmospherically it's pretty cool, there's plenty of flavor text to find and read, and they put some decent effort into sound effects. There's no music but to me that only adds to the sense of isolation.
There's a demo available, and while the $13 price tag may be a little steep for a game like this, I think it's worth the money. It's a little lighter on content than other classic roguelikes, but it's just as engrossing.
Tips:
-Use your knife early on to conserve as much ammo as possible, life withstanding.
-Try to be as accurate as possible, because again, limited ammo and panic fire can really drain your reserves quickly.
-Gadgets are the key to your long-term survival.
-Detector. Get one or die.
-Take your time and go slowly. Advancing too fast can see you end up with a ton of mutants and zombies on your tail.
-Be observant. Secrets aren't hard to find but they're easy to overlook, and they usually contain a lot of stuff you'll want to get your hands on.