Best way to add a rotating banner ad?

[dzeanah]

I'm looking for tools to enable advertising on my forum, and have no idea where to start.

What are considered the best applications? Right now I'd be happy with swapping banner ads each time a page is rendered, but the ability to limit ads to subforums might be seen as desirable by some advertisers, and ads that were search aware would probably be cool too (though I don't know if such a thing exists for vBulletin).

Can y'all point me in the right direction? If nothing else a short-list of good applications (free to otherwise) would be a great starting point...