The free tool can be found here: http://try.powermapper.com/demo/sortsite.aspx
Invasion Power Board: 9%
Simple Machines Forum: 18%
I was blown away from the comparison between vBulletin and XenForo.
BTW, I am not affiliated with that site or tool in anyway. I found it via google, and like using it. Also excellent for debugging my template errors or mods. It seems to be very reliable, except for the "spelling errors" part.