vBulletin is the most popular forum software used for forums and it is the software that I use on my sites, such as Podiatry Arena and the Foot Health Forum. I get a lot of customizations done to them to give the site the feel and utility that I want and each site has different requirements. They are constantly updating the software and there is an active modifications community for extensions and themes.
The only downside to vBulletin is that because it is so popular, it is a target for spammers and we have had to put in place extensive automated systems to keep it under control and manual surveillance systems to deal with the spammers. It is a headache, but a cost of running a forum. If there was not this level of vigilance, then a forum will be easily ruined. Forum profile spammers are a particularly persistent bunch.
vBulletin does cost money and there is no free version. The cost is not that bad considering what you get. Despite that there is a thriving black market of hacked vBulletin, which costs us all in the long run. The fact that there is such a thriving black market just points to how good the software is. It is used by a lot of forums.
