Battlefield 1942 - When was the last time you played a WW2 shooter anyways?
Runescape — This game is a part of my childhood. It's an in-browser MMORPG written in Java. It is click based, and great for getting absorbed into. The playerbase is, in my experience, young (because kids can't afford nice computers and a monthly subscription). Runescape 3 (this version being Runescape 2) is coming out this summer. It's worth a shot.
Realm of the Mad God — This game is like crack. It is basically a side scrolling, free-direction multiplayer shooter, skinned over with classic rpg classes. Each class has special traits, like a Knight is slow but is a heavy hitter, and a mage is of average speed but can do range attacks nicely. Each class has armor. It's similar to Runescape, but there are no real quest. Oh yeah, permadeath. Don't worry, you'll get used to it.
Theme Park — Another game from my past. I always felt it was a poor man's Roller Coaster Tycoon, however I still enjoyed it. I think you can ride your rollercoasters in this one.
Desktop Tower Defense — This is the game that started the recent defense craze. Set up your units in formation, and have them attack random mobs that roll through the play area. It starts incredibly simple, but scales up. If you want some quick tips on how to get good, play a couple of multiplayer games. You'll get your butt handed to you, but you'll learn quickly how to make an efficient grid, at least for single player.
FreeCiv — Played it a long time ago for about 5 minutes. Had fun, but couldn't figure it out. It's a classic, however, so give it a shot. Especially if Sim City has got you down.
Gunbound — This old game is basically multiplayer Worms. Do you remember worms? You moved your guys on a 2D plane and shot rockets at each other. It was fun. This game is pretty alright, from what I remember.
Canabalt — All it takes to play this game is one button. Despite it's simplicity, it's pretty exhilarating the first few times you play it. It's a browser game, so give it a quick look.
Knytt — One of my favorite pc games. This is a artsy 2D platforming game, before there was this influx of art games. Journey reminded me of this game. You crash on a seemingly barren planet, and you have to collect parts to fix your ship to get home. All you can do is run, jump, and climb, yet it all feels responsive and just the right amount of floaty. Play this game to relax.
N: The Way of the Ninja — This game has Ninja in the title for a reason. When you know what you're doing, you can fly through this platformer's maps with incredible ease. I'm bad at platforming games, so I never got this game to go as smooth as it was design, but I still enjoyed it. I believe the basic version of it is browser based; give it a shot!
Nethack — Played this a long time ago. It originally uses ASCII characters, but I had to use a tileset. This is a rogue-like, one of the royalty of the genre. It has a lot of classes, and some unique situations. It is slow going at first for a newbie, and has a steep learning curve (please please please use a tileset), but I've sunk a few hours into this game that I don't regret.
Dwarf Fortress — If you're in /r/Games and you haven't heard of Dwarf Fortress by now, you're messing up. This game is just... look, go over to /r/DwarfFortress, youtube some videos, download a newb pack (It'll replace the ASCII with tiles) and get playing. This will adequately explain what your'e about to get into. This game's randomness makes simply reading about others' playthroughs entertaining.
Cave Story — This is seriously my favorite platformer in existence, and will always bee in my top 25 games. This game is brilliant. It's a metroidvania style game, but it feels sooo good. The controls are pitch perfect, the music is just too catchy, the pixel art is gorgeous, the plot if humorous but dark, and the whole experience is simply fun. It is an absurd amount of fun. It was made all by one guy, but it feels professional. You can buy it, and I'd suggest it as you'll get updated music and skins, however playing the original version will not put you at any disadvantage (it's how I prefer to play (I havne't played the 3D version yet though, just the Steam version)). PLEASE PLAY THIS GAME!!
Grand Theft Auto 1 and 2 — I played GTA2 for quite some time on PS2 and with this free version. This is a 2d, top down crime game. It's systems are far different from modern GTA games. At the very least, you should play this just to find out where it all started. I have some fondness for 2, and it had some music tracks that were surprisingly great for this old game. Play it, for science!
Beneath a Steel Sky — I'm playing this now. I'll get what is potentially the biggest turn-off about this game out of the way first: it's point-and-click. If that doesn't make you dash off in the opposite direction, this game is worth a try. I'm playing it now, and it is my first PnC game ever, and despite it coming out in 1994 I'm enjoying it. It sucks getting stuck, but we've long had gamefaqs to help us out of a jam. The music, story, and humor are all enjoyable, and I feel at the very least to play it once till you get stuck. I'm no good at figuring out what to do, but if need be I'll be using GameFaQs to at least experience the story. It's that good.
Narbacular Drop — This is Portal before Portal was Portal. These guys were hired by valve and now we all have the ever-wonderful phrase "The Cake is a Lie". It's short, however there are a few extra downloadable challenge courses. This game is a little easier than portal since it's easier to shoot through a portal, but give it a shot. Like those early GTA games, it's interesting to see a game's roots.
Slender — Never played it. Watched some let's play of it. Creepy shit, guys.
QWOP — Only played it once, mostly just watch people trying to play it. I may just have a weird sense of humor, but this game is hilarious and has left me in tears a couple of times. Please, just play it without reading up on it. It is simply ridiculous. I'm not even going to tell you what it is, you should know by now.
DC Universe Online — Played the PS3 version. Lots of fun. I think it's better via controller, but it's harder to communicate on PS3 (idk about mic settings, but using an onscreen keyboard is no bueno). This is basically the only super hero/villain MMO game still running after City of Heroes shut down, so give this one a shot. Yes, the ps3 version is free too. Unfortunately, it's huge; make sure you ahve the room.
Gotham City Imposters — Team Fortress 2 with a Batman theme. It's way more fun than it sounds.