Here are lists of common ports in use on the internet:

Port Number     TCP or UDP               Name Description
20/21              TCP            FTP File Transfer
23                   TCP            telnet Console access to remopte computer
25                   TCP            SMTP Send e-mail
80                   TCP            WWW Web browsing
110                 TCP             POP Receive or check for e-mail
119                 TCP             NNTP Usenet News Groups
554                 TCP             rtsp Listen/watch audio/video content
1755                TCP            Windows Media Listen/watch audio/video content
5501                TCP            Hotline File sharing
6112                UDP            Starcraft Networked computer game
6346                TCP            Gnutella File sharing
6688/6699        TCP            Napster MP3 music swapping
7777                UDP            Unreal Networked computer game
8311                TCP            Scour Scour Exchange file swapping
27005-27035      UDP            Halflife Networked computer game
27901-27910      UDP            Quake2 Networked computer game
27960                UDP           Quake3 Networked computer game
69709-69720   UDP          Real Listen/watch audio/video content