I myself have always made it a practice to leave my URL at the bottom of my messages at various forums, mainly with the goal of building link popularity with Google. However, extremely few of the forums I've participated in had much if anything to do with my affiliate sites, so I may not be benefitting as well there regarding link popularity. They were mainly tech support forums or related to some hobby or interest of mine.
Yes, nearly all forums have very strict rules against commercial spam, but fortunately, none have a problem with leaving your URL at the bottom of your page. I have never added any promotional text along with it, out of concern it might violate the rules or make enemies with somebody.
Fred
http://www.internet-marketing-ideas.com