:Net Forum

[rpcarnell]

PHP has a forum called PHPBB, which may have its flaws, but at least it is free.
There's also OSCommerce.

What are the ASP.Net or ASP equivalents of these CMS's, if any?

[chazzy]

maybe DotNetNuke?

[LochDhu]

Yet Another Forum:

Open Source written in C# with SQL 2005 backend.

http://www.yetanotherforum.net/

I have deployed and used it in the past and it works great.