Danc of Lost Garden (a pretty good blog on game design) has some of his Tyrian
sprites and tiles up for grabs and usage. They're a weird size (something like 24x28, I thinks) but might be usable by someone willing to work up an alternate display system. Regardless, they're realllllllllllllllllllly good examples to learn from.

http://lostgarden.com/2007/04/ free-game-graphics-tyrian-ships-and.html