Blackhole is on sale DRM-free in Humble Store for ~€3-5 (
Complete Edition).
It's a quirky platformer with an entertaining storyline and narrative attached to it. Personally I don't play that many platformers, but this one really caught my attention, it seemed fun and humorous ... and very sci-fi.
It's a really tough game, some levels took me an hour to figure out, but as I said I don't play that many platformers.
The whole game took me ~30 hrs to beat, based on the Complete Edition, which gives you extra levels.
<span class="bold">SBX: Invasion Base Defense</span> on IGS, DRM-free for $2. It's a quirky mix of Tower Defense and classic space shooting, even the sound effects reminded me of the old stuff I used to play on the C64. It appeals to me for the same reason as the Creeper World and Particle Fleet games; they give lots of freedom in how you play the game, how you solve the problem.