Garry Newman, of Garry's Mod fame, has released the latest version of Facewound, a side-scrolling shoot 'em up. And by latest he means 3 to 4 years old.
Despite being old, unpolished, and still totally unfinished, the game is still surprisingly fun, with plenty of weapons to use and monsters to kill. There's even a level editor included (to create your own levels, duh).
Hint: press '~' then type in 'giveall' to receive all the weapons in the game.