Color Pencil Art Challenge Into colored pencil art? Check out this challenge?
DigitalPhotoContest.com An opportunity to showcase your digital photos and win prizes. Includes a forum.
Illustration Friday An illustration challenge site. You submit the link to your image based upon the given topic. New topic every week.
Internet Raytracing Competition Not for Photoshop, but for 3D and raytracing software. Some very cool stuff here to see.
myJanee.com Art Challenge Online art contests come and go, but this one's been alive and well since early 2001! Challenge is heald "almost monthly," entries are anonymous till after voting. Includes a forum.
PhotoshopContest.com Post your interpretation of their photo on your website and link to it through their Featured Photo section. Many entries are sexual or violent in nature.
Retouch Pro Not a contest per se, because there are no winners or losers. Entrants perform a restoration of a given photo and then share their technique. There is a related forum.
Worth1000 Many contests to look at or to enter. Forum, tutorials.