Rebrickable will be hosting a month-long competition to encourage the submission of some great new MOCs. The nature of Rebrickable is to promote re-using your LEGO parts to build new stuff so this competition will focus on Alternate Builds. An Alternate Build is a MOC that uses only the parts from a single official LEGO set.
I've been looking at options for a forum for ages but didn't find anything I liked 100%. However, the time has come to "just pick one already!" and so Rebrickable now has a Forum section :) The existing comments on Rebrickable are great for specific sets or parts, but lack the flexibility of a general discussion forum.