PostNuke and PHPNuke are okay. They're sorta overused now to the point of where it's easy to distinguish them. ASP.NET has a nice ASP one that tekmaven uses on his site. Basically you're looking for a CMS (content management system). They're available in PHP, ASP, etc ... you just have to have support for a web dev language and database on the server's end (e.g. MySQL, SQL Server, etc.).
First i would reccomend seeking a forum/board for your community. Once you decide on a script, find a CMS that uses the same web language as your forum/board. This will help you in the long run to integrate the two together and allow your community to visit more often.