I really like this forum layout as well. It works well with the clean interface theme of the software.

I've used PhpBB and vBulletin extensively. PhpBB is good if you're looking for something free but at the same time have some extra time to work with it. You may have to look thru some less than precise documentation to do upgrades and add the modules you want and this can sometimes be quite frustrating. Not to mention, due to its popularity it gets hacked all the time, which is reason enough for me not to choose it for something important.

vBulletin costs a little and has great features out of the box and great support. You only need to look at cgsociety's forums to see that its been proven.

As good as both of those forums can be, I hate the default layout of clicking on the category, then getting a thread list, then clicking a thread to see a post, all the time switching to a completely different page. This layout feels so much less stressful to use. People should enjoy browsing the forum, not be annoyed by it. This one gets my vote.