Typhoon 2001
I just picked up the latest version of this Tempest clone via Retro Remakes. Every time I play one of these it reminds me just how perfect the original Tempest was, and this time Typhoon actually works, thanks to new Vista compatibility.
Tempest, for the Martians in the audience, was the classic 3D space invaders (only different) by David Theurer. The game was apparently inspired by a dream in which Theurer witnessed monsters crawling out of the ground - a pulsing, neon ground, presumably. Tempest has been remade constantly since 1980 and just doesn't seem to get old. Anyway, here's the direct link to Typhoon 2001. Go play.