There are millions of forums out there, tons of them on every topic. Most of them will let you create a signature which shows up at the bottom of any post you make which can contain text or links. This is a great way to boost your stes link popularity. Become an expert on your subject and contribute to the community through forums. With each post you make, you’ll get a link back to your site.
Be careful though, don’t post junk. Don’t make posts to a forum solely for the purpose of linking to your site. People don’t like it, it’s not considerate, and it’s the kind of thing that causes webmasters to take signatures out of their forum. When people start posting junk just to post it, the forum becomes less valuable to the readers, people stop paying attention, and webmasters take away signatures.