Search Engine or Directory for Forums

Are there any search engines or directories to look for forums?

[ControlEngineer]

At one time almost all forums (boards, groups, or whatever) were part of a larger service, such as Compuserve, AOL, etc.

Now there are vey many forums, such as this. I have a forum about my professional field on my site. And then, of course, there are still groups of forums such as Yahoo groups. The problem with so many forums is that it is hard for someone with a question to find a forum.

Are there any directories or search engines (or search techniques) for finding forums on particular subjects?

[mikefs]

directory.big-boards.com seems like a good directory of forums and it is organized by subject.