bbPress
bbPress is forum software with a twist from the creators of WordPress. bbPress is plain and simple forum software, plain and simple. It’s easy to use, easy to administrate, fast and clean. But don’t let its simplicity deceive you; underneath the gleam, it’s got some powerful features and is highly customizable.
phpBB
phpbb is an open source forum solution. phpBB has an easy to use administration panel and a user friendly installation process, which allows you to have a forum set up in minutes.