For those who want to find new forums to participate in, here's another way to find them: Look for online discussion boards dedicated to forum administration. Most of the members will actually have their own forums, which they list in their signatures.
Some of these boards have sections for the members to show off their sites or submit them for review, too. Here are two examples.
I think this is a good way to find new forums that haven't yet accumulated enough content to rank highly in the search engines, but are nonetheless well managed, and likely to be around for a while.