Another option, if you are willing to pay for a license, is XenForo.
www.xenforo.com
If you need just a simple forum system, it is probably "easier" to just go with a native board. But in my community, where the forum really "is it"...and has been for years...we need quite a few features. We prefer to be on the cutting edge in that respect, and XF is the only board I've found (out of perhaps 15 or so researched prior to making the migration from vb) that even comes close.
I'm a former vbulletin user since it's version 1 (over 10 years). Due to a variety of issues, we've been forced to look for an alternative forum system. XF is by far, the most modern, advanced yet simple forum system on the market today, and it's only getting better. It's developed by the same guys who started vb (back in the day).
Right now, there is a fully functional bridge between XF and Joomla 1.5. A modded version works for 1.6 and a more official version is in the works (members have posted their sites with 1.6 and XF and it works fine).
There simply is no other board that can even come close with XF. Far too many reasons to list here, I recommend heading over to the site to check it out.