W3c validator found 100 markup errors. See them here. If you are new to maintaining valid XHTML and CSS, wander over to W3Schools.com and check out some of the excellent introductory tutorials they have.
OK, on to the site. My first impression: everything is text. The layout of the site isn't bad, but it's very uninteresting. As a visitor, I would not feel inclined to stay on your site for very long. I would try adding some things to break up your content a bit so that readers don't feel like it's a chore to read your site.
One thing I'd change is your Forums and Contact links at the top. When people are using your main navigation (in this case, your tabs) to browse your site, they are expecting the tabs to take them to different pages. The Contact Me link should take the visitor to a new page, such as a contact page, where you can then provide an email link and/or other contact information. It can be aggravating to click a link expecting a page, only to have your email client pop up unexpectedly.
Next, your main navigation changes from page to page. This can be a bit confusing since more often than not, the main navigation usually remains consistent throughout a site.
The borders on your tables on the Contributions and Links pages look very plain. Try giving them a little more style with some CSS.
I do like the simplicity of the design and the color scheme works very well. A little bit of graphics will go a long way into making your site top notch.
The Beer Monocle. Classy.