I've been a student of this topic for several years and have a few chapters in a book done.
The classic options:
A) traditional advertising in the form of banners or text ads.
Upside: :) if you are in a lucrative space, ad sells can account for much more than is commonly believed.
Downside :( from a community standpoint, banners can be very distracting. Most forums are hurt by banners.
Advertising - even text ads - will almost always hurt a sites credibility.
B) Sell subforum sponsorships with banner or button for product on that topics page.
Upside :) if you have great targeting, you can charge significang amounts for sponsorships of forums.
Downside :( credibility goes out the window. You simply can't have reliable, believable discussions about said products when that products name is on the forum. All such forums are eventually doomed. Forum or topic sponsorships are a slow cancer on any forum. Stuff like this grows like mold all over your forums.
Upside :) maintains integrity of public side and creates a builtin quality assurance barrier.
Downside :( only one site on the net has done it somewhat successfully.
D) Donations/Tip Jar.
Upside :) Low overhead. Mantains sites integrity.
Downside :( You have to have a very faithful following with mass traffic to make it pay I think.
E) Combinations of the above.
Upside :) You can work a little wee bit of the above and still maintain your full independence.
Downside :( A pr challenge. It is having the worst of both worlds - tough tough for you to explain to your members.
F) Nontraditional creative monitization ideas for a forum, but traditional ideas for the rest of web.
Things such as: classified ads, our flirtation with WebmasterWorld Answers in 1999, or auctions of products related to your topic. There are many ways to use traditional money makers in the forum space if you dig deep enough.