I think Markdown formatting would be a great addition to the forum. Unlike BBCode or QQCode, it flows when writing. You can format on the fly without losing your thought or flow. It's wonderfully simple and very readable. I don't think it's as feature-rich as other formatting methods, but it gets the job done. I don't think I've seen a forum use it before, but it would have a positive impact on any site's use.

It's free and open source.

[url=http://daringfireball.net/projects/markdown/]Official site[/url]