This is always a good question to probe how often a candidate deals with specific higher-layer protocols.
There are too many ports to mention, but the critical ones are the following: FTP (20-21/TCP:UDP),secure shell (22/TCP:UDP), Telnet (23/TCP:UDP), TACACS (49/TCP:UDP), DNS (53/TCP:UDP), SMTP (25/TCP:UDP), TFTP (69/UDP), HTTP (80/TCP), SSL (443/TCP), POP3 (110/TCP), RADIUS (1812-1813/UDP), SNMP (161-162/UDP), BGP (179/TCP), LDAP (389/TCP:UDP), RDP (3389/TCP), and IKE/ISAKMP (500/TCP:UDP).