Many of the writing competitions are offered by a specialized sub-group of the American Bar Association. I've linked the ABA's site on the side bar (it's on the right side of this screen, under the heading "Writing Competition Website Collections"). The ABA site also is linked through this post.
If you're making some last minute changes to your semester, choosing a research topic for an independent study, law review or law journal effort, take a look at the ABA site. The array of competitions offered in the various sub-specialties in law can help you with writing topic ideas and also may give you the extra inspiration to take that seminar you've been looking into.