I think a project competition with prizes would go down great. You already have a voting system in tact. But unfortunately, it's not such an obvious thing to be able to do. (This is one of the reasons why few threads actually have votes) The voting needs to be more clearly visible somewhere. It needs to be seen at a glance.

The other problem is that you're allowing people on the site without having to create an account. If you made it mandatory to sign up for membership after visiting the site more than a couple of times - this would possibly improve interaction. You could also enforce voting to some extent too.