You could use IRC. Or learn PHP or Java or C# or whatever to program your own forum software, seeing as vBulletin requires a license fee (I think, check their website though).
Free servers are moderately easy to find though: http://www.google.com/search?hl=en&q...=Google+Search
***Edit To #3 & #4***
3. Not that many members and some advertisments for them
4. Failure to exceed profit margin