http://www.dnp.in (domain discussion forum). We just started a referral contest, which will decide who has referred the most new members to our forum. The winner will be decided on 11/26, and will be rewarded $20. See http://www.dnp.in/showthread.php?t=488 for detail. Also, we have other posting contests with prizes of PR6 links. Feel free to check out our forum if you are interested in our several forum contests.
Referral comps like that usually only lead to people signing up then running away.. My opinion anyway.