I don't know about free forums, but a lot of paid membership sites at a wide array of price points with a forums format make very good money. I know of a couple that make $20,000 to $100,000 per month.
Ok I see, but I wanted to know about the free ones, like those that earn a lot with adsense or cpm ads
I would imagine that Young Entrepreneurs is doing pretty well for themselves with such a large user and visitor base. They've also got a lot more ads on the site.
I am sure there are quite few making some money... it depends on the numbers you get signed up but sure you can make cash.
It depends on the numbers as well as the members. If the forum is attracting folks who are typically "ad blind" it will be much harder to monetize.
I would guess forums like vn7.com, sitepoint, and digitalpoint forums are all doing good. You just have to find the right niche.