Well, first about ports: as have been stated before, there´s a comprehensive list o well-known ports and protocols related to them on IANA (Internet Assigned Numbers Authority).
But it doesn´t mean a program can´t listen on another port, which would be useful to be able to work through some firewalls. A Trojan may use a ftp port, for instance. (21). If you have access to a *nix like so, you may know the inetd. It controls a lot of servers, and reading the lines of inetd.conf (its configuration file) you may where are the programs running on each port (or that may run, since most are commented). Having a look at the line about systat, you may notice this command line : ps auxx . Try this on you machine and you will know how systat works. At least one implementation..
Ah, a nice substitute for telnet as a general "connect-on-everything" is netcat. I think it is way better than any telnet client for several situations. Well, one problem is that it doesn't filter ansi color code, so its awful for playing MUD.. . But its features are pretty nice .
And you may want to know about ssh, and use it instead of telnet when you want access to another computer, since it offers you encryption of data. Passwords are easily captured in a telnet session, for they travel in plain text. hmm and emails are usually sent "naked" .. another thing to worry? Probably.