All of the below forums allow you to advertise your site(s) using anchor text of your choice within the signiture area. This allows you to benefit from contributing to forums and is a handy and easy way to build links. Below I’ve included general SEO forums, as well as more local examples towards the end. I’d advise you to search for sites in your niche as this will likelly provide more benefit to you. Also, do not spam. Add something of value, and you’ll receive something of value back 
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SitePoint
Warrior Forum
Digitalpoint
SEOers
V7N Forum
Blogger Talk
SEO Round Table
SEO Chat
SEO Forum
Web Pro World
UK Business Forums
Sheffield Forum
Entries (RSS)
As an admin at SEOers.org, whilst signature links are allowed (infact actively encouraged), as a result we are very strict on spam postings, not only do we have numerous automated anti-spam features we also have an active moderator team and will see through any veiled spam attempts.
Do you think I should update the post to emphasise the community and learning aspects to these sites? I was trying to get across the existence of such sites, as well as regional and niche specific forums, rather than the act of spamming, which will of course often result in users being banned.
Sadly there will always be users who abuse the system, no matter what you, i or anyone say or do.
I think forums offer much more than just a link gaining opportunity, users who actively contribute in forums (related to their niches) can actively gain from not only links but also gain potential visitors, customers etc.
Classic example, is yourself, i visited this blog originally after seeing your posts at http://www.seoers.org, and then secondly after seeing this blog post in the referral logs.
I have installed DoFollow on all my blogs hopefully this will attract more visitors and more links for everyone.
- Jack