The ability to gain near real time answers to your SEO questions makes SEO forums extremely valuable. I have my opinions about which ones are best, and likely you will have your opinions about which ones you will like best.

A good forum has a good sense of community, but different SEO forums have different goals. Generally I support the fact that good SEO is generally making good ideas that would want to spread and / or gaining a large quantity of links into a website. The forums that allow open discussion of and support effective SEO ideas are my favorite.
You will learn a bunch of your SEO knowledge from personal experience, but forums will also help speed along the learning cycle.

Free: Review of and links to most of the major SEO forums (and many smaller ones too). Most SEO forums are free also. (www.seobook.com/archives/000161.shtml)