Keep an eye on a fairly new forum called bzimage.org It's growing in popularity every day. It's nowhere near as big as DP but I gather in another years time it will be up there with the best of them.
Sitepoint.com is a tough competitor for DP,although SP provide nice knowledge too,i like staying with dp .
I have used many forums and DP is the best... there are some really popular forums that are just full of Nazis you can't do any sort of promotion at all or post a link or anything. Some forums are way too strict. DP has a good mix I think and it has worked well for me and obviously a lot of others!
I have to agree with you on that one, the marketplace at SitePoint is by far the best, as in design and how it works, I don't really know about the reliability and scam rate, etc.
dp is prob the best one but right now its starting to get over run with scammers and SP ,v7n , WF are up there.