Digg.com, one of my favorite sites, has a link to this site which lists TCP and UDP ports that many applications use. Why would you need such a list? -- well, one good reason is so you can open up (or close) ports in your firewall to allow (or prevent) access. It lists all of the standards, like SMTP (25) and TELNET (23) but missed SSH (22) and Exchange (a whole range of ports). It's strength, though, is that it lists a lot of games.