How complex do you want to get with the ads?
Rotating Banners?
Subforum specific advertisement?
Clickthrough tracking?
Content-specific advertisement?
End-user administration?
Hit based display limitations? (I.e., ability to sell 10,000 views or 100 clicks, but only display the ad for that many hits)
As Charby mentions, you can hack it into most existing forum software, even if it doesn't initially support it. Since most forums use a framework, a decent coder could probably set up the most basic banner system for between .25 to 1 hour worth of work, depending on the forum code.