Robokill: This is a phenomenally well-done Flash game. It’s a shoot ‘em up, but it’s almost perfect — one of the best Flash games I’ve ever seen.
They give one “episode” for free, comprised of four levels, and that’ll be enough to keep you busy for a couple hours. If you want to finish, it’s $9.95.
Great graphics, great sound, great controls make for lots of re-playability.