Some relevant port numbers:
7		echo
17		qotd			quote of the day
19		chargen			character generator
20		ftp-data		file transfer protocol
21		ftp			file transfer protocol
23		telnet
25		smtp			simple mail transfer protocol (send mail)
53		dns			domain name system
69		tftp			trivial file transfer protocol
70		gopher
79		finger
80		http			hypertext transfer protocol
109		pop2			post office protocol (obsolete)
110		pop3			post office protocol (receive mail)
113		auth			authentication/identification
119		nntp			network news transfer protocol (usenet)
137		nbname			netbios
138		nbdatagram		netbios
139		nbsession		netbios
443		https			http secure
4000		icq			I Seek You (communications network)
4995		vpn			virtual private networking
4996		vpn			virtual private networking
4997		vpn			virtual private networking
6667		irc			internet relay chat
12345		netbus			trojan horse virus
12346		netbus			trojan horse virus
20023		netbus pro		trojan horse virus/remote administration tool
31337		backorifice		trojan horse virus