Phorum requires PHP 3.0.9 or higher available from the PHP site or if you are leasing web space, your provider may already have it installed. The databases the current release supports includes MySQL, PostgreSQL, Sybase and MS SQL Server 6.x. You can get PostgreSQL at the PostgreSQL Home Page. These two are OpenSource. Just download, decompress, configure and go.
Lite version is Free.
These two are OpenSource.