One great free software is
http://www.usebb.net - you can set it up so that it requires new registrants to answer a random maths question, so that keeps out the hordes of spammers who target some of the other softwares. It's a fairly simple installation, needing 1 mysql database ....
I do like the vbulletin that this forum uses; but you either purchase a license or lease it on an annual basis
Hope that helps
Margaret