It's often very useful to have a list of common ports. Here are some ways to get that information.
Internet Assigned Numbers Authority Port Numbers ~ This is the canonical, official list. IANA is the organization that actually assigns these numbers, so this is really the place to look. http://www.iana.org/assignments/port-numbers (For the complete Directory of General Assigned Numbers, which covers just about everything you could ever imagine, see http://www.iana.org/numbers.htm)
If you're running Windows NT, 2000, or XP, this file will give you a nice list: C:\WINNT\system32\drivers\etc\services. If you're running Windows 95, 98, or ME, look for this file: C:\Windows\services.
If you're running Linux or any *nix (including Mac OS X), look in /etc/services.
Ports below 1024 are defined by the IANA. They are called the Well-Known Ports.
20 FTP data (File Transfer Protocol)
21 FTP (File Transfer Protocol)
22 SSH (Secure Shell)
25 SMTP (Send Mail Transfer Protocol)
53 DNS (Domain Name Service)
68 DHCP (Dynamic Host Control Protocol)
80 HTTP (HyperText Transfer Protocol)
110 POP3 (Post Office Protocol, version 3)
115 SFTP (Secure File Transfer Protocol)
119 NNTP (Network New Transfer Protocol)
123 NTP (Network Time Protocol)
143 IMAP (Internet Message Access Protocol)
161 SNMP (Simple Network Management Protocol)
194 IRC (Internet Relay Chat)
220 IMAP3 (Internet Message Access Protocol 3)
389 LDAP (Lightweight Directory Access Protocol)
443 SSL (Secure Socket Layer)
445 SMB (NetBIOS over TCP)
993 SIMAP (Secure Internet Message Access Protocol)
995 SPOP (Secure Post Office Protocol)
Ports between 1024 and 29151 are known as the Registered Ports. Basically, programs are supposed to register their use of these ports and thereby try to be careful and avoid stomping on each other. Here are some common ports and their programs.
1243 SubSeven (Trojan—security risk!)
1352 Lotus Notes
1433 Microsoft SQL Server
1494 Citrix ICA Protocol
1521 Oracle SQL
1604 Citrix ICA / Microsoft Terminal Server
2049 NFS (Network File System)
5010 Yahoo! Messenger
5190 AOL Instant Messenger
6000 X Windowing System
6776 SubSeven (Trojan—security risk!)
7070 RealServer / QuickTime
27960 Quake III
31337 BackOrifice (Trojan—security risk!)
For a complete list of Trojans & their ports, see http://www.simovits.com/nyheter9902.html.
Ports from 49152 to 65535 are known as Dynamic or Private Ports. Anything goes here!