Forum signatures crowded with links, banners and all kinds of things linking to external pages seem to be very popular now. If you're active on a forum that has a lot of members and add a link to your site in your signature, chances are you will get a few new visitors. As long as it's not forbidden by the site's rules (some admins do not want members placing external links on their sigs), it is a very easy way to get people clicking on your site's link. The more you post, the more hits you'll get!
Have you ever linked to your site through a signature in a site you're active on? Have you ever paid someone else to do it for you (by "renting" his signature space)? Or perhaps rented your own signature?