May 4th, 2008  
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Getting ads for related services/products to your forum can be an alternative to Adsense.
CJ.com, Linkshare.com etc. have many good advertisers to choose from.

Another thing I've seen is software/hardware (or similar) reviews with affiliate links to the product(s) they write about.

If you've got a very popular forum you can charge to get access to certain areas of the forum, like buy/sell/trade areas, "special" areas for "Gold" memebrs etc..