There are many forum scripts out there, but my favorite is SMF. Sure VBulletin has a lot of features, and skins, but it's not free. On for those that plan to have a forum but are on a budget, SMF gets the job done.

One of the biggest disadvantages is probably the limited # of skins available (compared to phpBB/vBulletin) However, despite that, there's still a decent # of skins available that should suit everyones need and may only require small modifications to properly suit you.

To get all the themes: http://themes.simplemachines.org/

In the event that you had another forum which you'd prefer to use SMF on, you can convert it as well: http://www.simplemachines.org/download/?converters

**Do a backup before doing this! And follow the instructions**

SMF has a built in SEO friendly feature, however, it's not enabled. You must enable it yourself, log in as an Admin and check it on the Server Settings panel.

There are also bridges that you can use to integrate SMF into mambo/joomla (as well as others) http://www.simplemachines.org/download/?bridges

The system requirements are available here: http://www.simplemachines.org/download/requirements.php

*Note* The links here point to the SMF site, which mentions these things, however some users, especially new ones, might not understand what each function is for.

And if you want to implement ads easily then use this: http://mods.simplemachines.org/index.php?mod=255

You may have to tweak it a bit if your not using the default theme. Some minor PHP knowledge is required for that.

Source: Myself