Nuclear Throne shouldn't be as fun as it is, given how often it kills me (goddamn that 5-3 boss). There's something very endearing about its animations -- everything feels (and sounds, somehow) kind of squishy, which is good for a radiation/mutation themed setting. If I had to pin down one mechanic that hooked me, though, it's the weapons -- both the huge variety, and way it forces you to improvise as one runs out of ammo, and you toss it and pick up the next one you find. It's super fun and fast, and surprisingly challenging (I have yet to reach the final boss, even). I got it through a humble bundle, but it's on Steam for $12.
Atlas Reactor is something I just picked up, having read about it over on Penny Arcade. I have a feeling I'm going to be playing a lot of it. This is a weird game type -- it's a turn-based, player vs. player focused team arena battler. Folks have described it as X-Com meets DOTA. For me, it's a fast (20 second turns), competitive (tactics, bluffing, and reading), team-based game (silently blame teammates when you lose!), with a roster of characters to learn and master, and one that I can play without ruining my hands at just a few clicks per turn. Also, surprisingly good aesthetics and sound. It's in open beta right now, and will have a "free mode", whatever that means -- to play the full thing, you'll eventually need to make a one-time purchase of $30 ($20 before it launches in October).
For Honor in a nutshell is AAA polish applied to what seems like a very niche market: multiplayer melee combat. No guns, very little ranged abilities. This is a game about charging, bluffing, thwacking, blocking, and dodging. I played the game first at PAX a few weeks ago, and was impressed by the weighty feel of the game. It reminds me a bit of Soul Calibur, but even moreso of Jedi Knight 2, an old game that had an amazing multiplayer dueling aspect that I believe inspired another melee combat game, Blade Symphony. These kinds of games combine a very high skill cap with surprisingly entertaining combat. Combat is typically more methodical, elegant, and flashy -- which makes for great dramatic fights and crazy comebacks. For Honor will probably cost a lot of money when it comes out, but you can sign up for the alpha/beta here.