Perhaps the most popular of all discussion boards - not least of which because it's FREE!

phpBB is a highly versatile product with a huge range of features. It has suffered on and off over the years with security issues, but those seem to be ironed out with the latest versions. As one of the most popular pieces of software around it is bound to come in for some attack, but don't let that detract from a superb product with a huge variety of add-ons from a worldwide community of developers and users.