For this occasion, the organizers of the Humble Indie Bundle have dropped the "bundle" shtick -- at least for the time being -- to bring us the Humble Voxatron Debut. Similar to past promotions, this new game is DRM-free, cross-platform, and you get to name the price. Suhweet.
Your purchase isn't for a finished product; rather, you're getting future content updates, as they come out, for free. Not everyone is cool with this approach, but I'm quite fond of it myself. I'm also into voxel-based games inspired by Robotron. What else might be in store, given that this is a two-week sale?