Found this on bungie.net:

For Halo:
80 (Transmission Control Protocol [TCP]) - Halo looks to see if there is a Halo update available. If there is an update, Halo downloads the update through this port.
2302 (User Datagram Protocol [UDP]) - the game host listens on this port.
2303 (UDP) - the game client connects to the game host through this port.

For Gamespy and Arcade Application:
6667 (IRC)
3783 (Voice Chat Port)
27900 (Master Server UDP Heartbeat)
28900 (Master Server List Request)
29900 (GP Connection Manager)
29901 (GP Search Manager)
13139 (Custom UDP Pings)
6515 (Dplay UDP)
6500 (Query Port)

May or may not be useful.