The best open source forum software (for the core) is MyBB.
http://www.mybb.com
IMHO, if you spend time with MyBB it is almost impossible to consider SMF or phpBB as serious contenders.
ps.
Building a quality forum from scratch is almost as silly as working to eliminate a bounty worth over $70,000.![Cheesy](https://bitcointalk.org/Smileys/default/cheesy.gif)
http://www.mybb.com
IMHO, if you spend time with MyBB it is almost impossible to consider SMF or phpBB as serious contenders.
ps.
Building a quality forum from scratch is almost as silly as working to eliminate a bounty worth over $70,000.
![Cheesy](https://bitcointalk.org/Smileys/default/cheesy.gif)
The goal isn't to eliminate the bounty.
The goal is to break it into smaller pieces which individual developers can actually reasonably achieve.