Conceptart.org runs a series of weekly contests online. They have several categories: Creature designs, Character designs, Environment designs, Industrial designs, Illustrations, Comic Panels, and a teen only category. There are no prizes and the winners are determined by popular vote, but the site is frequented by many working professionals and winning can be good exposure. Beyond that the contests represent a good means to practice working to briefs and with deadlines. You can potentially also get valuable feedback on your work.
Entering these on a regular basis after college may be a good method of keeping my skills honed while I search for work.