Civilisation 4 ports
There is an absolute epic on which ports need to be opened for Civilization 4 to play multiplayer.
Although it seems massively over-kill for a large number of people. Most people don't block by port outgoing, but will filter by application signature (if at all). So the only port that needs to be opened is for 'hosting'.
This appears to be 2056 (Microsoft DirectPlay) on UDP. I've been happily playing with only this open. I think it gets more complicated when more than 2 people are playing as it seems to be peer-to-peer in nature. I'll update if there are any other considerations when I play with more people.