MyBB is my favorite. It feels a lot like vBulletin (some people have made skins that look exactly like vB), but is open source (aka free).

Quote:I don't know how Version 3 is, but I know Version 2 was known to have security issues, quite a bit.

phpBB2's security issues caused hosts to completely ban it from being used on their servers. I have no idea how phpBB3's security is, as I've never really used it (outside of playing from a few SVN checkouts WAAAAAAAY before it came out).