Minecraft wiki entry for Setting_up_a_server, associated text: 21 (Used by most FTP Servers)
22 (Used by Secure Shell daemon)
25 (Used by Mail Servers for SMTP)
53 (Used by DNS Servers)
80 (Used by most Web Servers)
110 (Used by most Mail Servers for POP3)
115 (Used by Simple File Transfer Protocol)
143 (Used by Mail Servers for IMAP)
443 (SSL port for Web Servers)
3306 (Used by most MySQL Servers)