Free Shmup Pixel Graphics
I just was browsing through my tutorial/template folders when I found Danc's huge load of free shmup sprites. They were ripped from Tyrian's source code and are free to use for games, prototypes, animations or other graphical work. The only thing Danc asks for is a back link and maybe a short notice about your project.
Enjoy them. The following two tile sets are just a small part of the whole collection:
Get Tyrian's Free Sprites (direct download)
Cheers,
Thomas