There are millions of Web based forums that range on topics from math to soccer. Many of these forums have strict policies on where you can link to externally and what you can or can not have in your signature file.
At DigitalPoint's forum, there is a thread that discusses the value of a sig link.
There is no doubt that signature links on some forums count. Depending on the forum's policy, (1) the forum might allow clean links to external sites, (2) the forum might run an external link through a redirection script, or (3) a forum might not allow for any or a very limited type of external links. This entry is not about which SEM forum allows for which type of links, I am not looking to make enemies.
I would just like to point out that after this week's Google PR and backlinks update, SEO Chat Forums finally reached a PR8. I spend most my time posting over at SEO Chat, since I am a moderator over there. Google had some sort of penalty on SEO Chat, the reason was unknown. It seems that penalty has been lifted about two updates ago and now Google finally crawled the forum threads. The sites in my signature file at SEO Chat have gained about 350 links each. The PR of those sites went up slightly but the number of links increased dramatically.
So what is the value of the links at your favorite forum?
declaimer: I am not advocating joining a forum just to increase your link popularity. Join a forum to get the best advice possible from people you like. Each forum has its own culture, make your own decision.