Litespeed Webserver (http://www.litespeedtech.com) has shown that it can more than adequately run vBulletin with high concurrency rates on my system with speed enhancement, even over Lighttpd. It can use a custom API (LSAPI) which you can choose to compile into PHP to gain even greater speed enhancements. It cut my server's load in half, and thus I recommend at least checking it out...the standard version is free, and other than a 300 concurrent user limit (ie, transferring data at once) it has no limits. :banana:

The versions beyond standard do cost a bit of a fee, but if you're a large forum making $20k monthly, and have need for a multicpu (ie, quad opterons) it shouldn't be a problem. It's custom SAPIs for PHP, Ruby, and Ruby on Rails was among my points for switching from Lighttpd. Either way, I like both Lighttpd (which seems to be popular here) and Litespeed, but if nothing less give it a run.