Found via the IndieGames.com blog, if you follow indie developer Rake in Grass on Twitter, they'll send you a code for a free copy of their game Larva Mortus (I got mine just a few minutes after following them). They'll be doing this promotion for another week, so get your code now.
The game is pretty decent. I haven't played enough of it yet to say whether the story is compelling or not, but so far it's really just about the action, not the story, which is fine. It's a top-down shooter where you move with the arrow keys and aim and shoot with the mouse (do developers ever consider us poor laptop-with-trackpad users who have to go hunt down a mouse?). The tone of the game is nice and creepy, with nice music and graphics to support the spooky atmosphere. Once you've killed all of the monsters in a room, it's "cleared", and you are safe in that room from then on. This means you end up spending a decent amount of time just sitting around in cleared rooms as you heal up (I ended up advancing my regeneration stat just to make that time shorter). Maybe it should let you heal faster if you're in a cleared room or something? Anyway, it's a minor quibble. The variety of weapons is nice (I haven't seen them all yet, but you start out with a sword and pistol, and you end up picking up a shotgun, a crossbow, a machine gun, a stake, and a cannon(!)), and it's decently challenging on "normal" difficulty. I definitely recommend it for free, and it's probably worth checking out some of their other games.
Edited to add: Available for both Windows and Mac.