Accepted file formats are .mov, .mp4, .swf and, most relevant for us pixelers, .gif.
From the guidelines: "animated gifs should be no wider than 640pixels and no taller than 600pixels."
Most submissions are videos, but there are always a good number of great gifs too, even a few pixels. The April 2013 challenge was just GIF.

The current challege theme is Empathy, deadline is May 17.
Prizes: "Selected loops will be posted within 7 days of submission for everyone to enjoy, comment and rate. The top 100 loops will screen in our host cities."

Let's submit some pixels!