PunBB is really nice. I've been very pleased with it (as someone who has used vB, phpBB, and SMF as well). If all you need is something lightweight, it's great. Light on moderation abilities, and takes some hacks to give it things like PM capability... but it actually scales pretty well.
Check out
http://punbbig.shacknet.nu/ (which is actually down as I write this)... it lists the largest PunBB installations out there in terms of users and posts.
One of the nicer ones:
http://www.macaddict.com/forums/
And here is a rather simply skinned installation I have running:
http://forums.awscms.com/index.php
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