I have had two:
Let's Sermo: $300 worth prizes with $75 in cash included. For the prizes which were mainly banners and link backs. This contest was a HIT. It got blogged a lot and I got over 100 comments on that blog post and other posts too got many comments.
Webmaster Peers: $1500 worth prizes with $250 in cash included. The result was quite good, but sadly not many people took part. Just a few members were in it.
Have you ever held a contest? How was it?