SMF is an open source forum system. What I really liked most about SMF is the ease by which you can upgrade to the latest version whenever it is out. In other open source solutions, it is often not easy to upgrade to a newer version. You usually have to download the new version, upload it to your site, then fix some stuff that could have been broken due to the upgrade.
With SMF, each time a new version is out, all you have to do to upgrade to that latest version is to click on a single button inside the admin area of the installed forum system itself, and the magic happens, it self upgrades to the latest version, with no problems taking place. For me, this is perhaps that brightest thing I found in SMF, specially after the headache I had trying to upgrade my Drupal to a new version, loosing my old data in the process.