Funny you should ask this, because I was just wondering the same thing not long ago. I did a Google search on the subject and, as Wycen pointed out, the only other forum besides EN World that I found is the RPGnet forums. There is a lot of talented competition there, but it is worth taking a look at. I also agree with Wycen about checking out individual publishers' websites. Some publishers are always accepting ideas from writers and have their guidelines posted on their sites. Just check out some of the publishers that post here, and visit their websites for more information.