Have you ever left a business card for your forum inside a book at a local library? Go door to door and leave the little door hangers advertising your webpage? Personally speaking, i've yet to do either, but that could change soon. I'm actually looking into doing the business card trick, I have family and friends who swear by it. It's quite simple to pass off as a "bookmark" and there's really no harm done. If you can match your target audience then there's a good chance you'll get traffic.
Have you ever tried either method? What results did you end up with?