Free games:
The MDickie "demos": They're pretty good for what they are and the time period they were made in. Pretty sure he was one of the first ever indie developers, so the sort of ego that comes off in his games is somewhat deserved. I wouldn't recommend spending money on them, but beating other pro wrestlers to death in Biblical times (And in prison) is a fun way to kill 30 minutes.
Toribash: Also a fun way to kill a few minutes, but it's only funny when you watch the replay and see what you did.
Pirated Project64 ROMS: If you can find out how the shit you can work Toad's Tool and Text Wrangler, Super Mario 64 becomes a sandboxy barrel of fun. Also, Doom's really fun to whip out every now and then. Can't say much for the goddamn keyboard controls.
Tricky Truck, Toribash, Goofball: All made by the same developer. They all have paid versions, but you're not missing much.
Warframe: It's a bitch to load at first, just like TF2, but I guess it's fun. Grinding never even feels repetitive, and the combat is fluid and satisfying.
Space Funeral: It's a JRPG, but it's really, really interesting and cool. Surreal as hell, too.
Earth Birth: Sequel to Space funeral.
OFF: In the same vein as the last two, but with a completely different feel and theme. The combat is roughly similar, but satisfying too. Interesting-as-hell story. Make sure you install the right fonts, or the English version will be hell.
Paid games:
Saints Row IV: A game that everyone needs to play at least once in their life. Fucking awesome. Gat out of hell isn't worth it, imho, but you can buy the cheap version with all the rest of the DLC too.
Skyrim: What good shit can I say about Skyrim that hasn't already been said?
Fallout NV: New Vegas is much better than fallout 3 from a story aspect. Security actually feels more competent than most other Bethesda FPSRPGs, the Black Armor is freakin' awesome, the DLC has a really in-depth story that wraps up REALLY well with Lonesome Road, and all manner of awesome shit. Fallout 3 has Katanas and a Lovecraftian DLC, though, so it's quite a hard decision.
Oblivion: If you mod it to shit and pretend the voice-acting breaks aren't there, (I reccommend turning the voices off in the options and doing your own in your head.) it's actually better than Skyrim, but if you mod it to shit, it runs much slower, and your friends will judge you for adding the big boobs and bikini armor mods.
Terraria: It's as hard as fucking shit, but building is kinda fun, I guess.
Minecraft: Hours and hours of fun. It gets boring unless you're competent enough to add mods and maps. But ever since Mojang sold out to Microsoft, it's been impossible to mod, so use earlier versions of it, if at all.
Jazzpunk: Fucking awesome adventure game by Adult Swim... Why do I like trippy things so much?