I've been addicted to the shareware version of Debris 32 for over a year. Debris 32 is like Asteroids: you pilot a ship and go around shooting large asteroids to break them apart and small asteroids to destroy them. Debris 32 adds mines, aggressive enemy ships, and power-ups.
It took me a month before I was able to beat the game on the hardest difficultly level, and I can still only beat it half of the time. The hardest levels are 4, which has a hard-to-destroy enemy base, and 6, which has crystals. Small crystals grow into large crystals if you don't destroy them quickly, and shooting a large crystal results in several small crystals.
The shareware version is addictive despite only having 10 levels. I don't know how to get the full version. The publisher (MVPsoft) no longer sells it and I don't know how to contact the author (David Bollinger).