I've bounced around a lot of different forums and online communities in my time and have seen a lot of different ways those sites have monetized themselves. The best method that I've seen so far is giving out a cosmetic badge for donations. It's like users are paying to help support the site and to have a few pixels to feel ever so slightly superior to the other users. The most effective way I've seen this implemented is by offering different badges for different donations amount (e.g. $5, $20, $50). A lot of users will donate simply to get the $5 badge, benefits to the site aside. Once they make a smaller donation and have a badge next to their username to remind them of that, they are far more likely to make a larger donation, if only to get another badge. I'm willing to admit that I've bumped up my donation to a site solely to get a badge, and I'm sure many other users would too.