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.

MGM out

Depending on you comfort level with scripting and forum upkeep I would recommend Invision Free.

http://invisionfree.com/index.php

It's really easy to upkeep and there is a lot of support. You can hack it to add chats and things like that pretty easily.