TowerFall Ascension [official site] is a grand game for friendly jostling on the couch with your pals as you try to murder each other (and in the game). Pling arrows, snatch powerups, catch arrows in mid air, and shout "OH MY GOD NO HOW DID YOU-!" - it's a right lark. Now you can make your palmurders a little more personal, as a level editor is out for folks who also own its Dark World expansion.
For folks who are new to chumicide, the game and Dark World are on sale for a bit.
Update 1.3 yesterday added the Workshop, letting folks make their own levels for Versus mode (not the singleplayer or co-op stuff). Naturally, the Workshop also uses Steam Workshop to share levels - the TowerFall Ascension: Dark World Workshop Workshop.
You'll also find a new variant - Infinite Brambles - a new stage element for custom levels, bug fixes, and other stuff in the update.
So far the update's only out for the Windows edition, but TowerFall creator Matt Thorson says its newness is coming to Mac and Linux "very soon."