I've been looking at several forums for a few of our internal sites that are lightweight, follow some resemblance of web standards, and built in light PHP or Ruby on Rails. PhpBB, vBulletin, and others just aren't what I'm looking for since each has a certain amount of vulnerability from hackers, are more succeptable to spammers, and are hard to configure or design to fit the rest of a web site.

A few that recently caught my eye were Beast and Vanilla. Beast is a small, light-weight forum in Rails with a "scary name" and a claim of about 500 lines of code. Now that's light! Vanilla is an open-source, standards-compliant, multi-lingual, fully extensible discussion forum for the web. It also comes with some pretty cool forum addons and extensions.

If you're looking for something small, lightweight, and extremely optimized, then either of these two should fit the bill. Since SEO Position and my other sites use Drupal's open source CMS, I might not completely rule out the Drupal vBulletin module (appropriately called Drupal vB) that should plug vBulletin right into Drupal.