First off, one of my very favorite events: The Supernatural Springfling
"This is a rapid-fire, double-blind, anonymous exchange challenge, in which you have a brief set of prompts, a week to create your fic/art/vid, and a lower AND upper limit on the size of your work." You aren't inclined to procrastinate or over-think stuff. I love the immediacy!
Secondly, an event that's technically new, but not really. Every year, ohsam does something to celebrate Sam's birthday in May, and this year we're collaborating with spn_littlebro on a simple little prompt challenge. Just slap down a couple words and get inspired. (There are tumblr options too, if you're more literate with that platform.) The event open March 25th, so do the spnspringfling first!