Instant messaging (IM) over the Internet has grown extremely in popularity the last few years.
If you get into the habit of "chatting" to people that way, you might find yourself hampered by the fact that some of the people you want to talk to use AOL Instant Messenger, some use MSN or Windows Messenger, some use Yahoo Instant Messenger, some use ICQ, while some use something else entirely.
You could have each of those four (or more) programs running at once so you are connected to all of them, or you can install and use Pidgin (formerly known as Gaim), which is a free, open-source program which will connect to all of those instant messenger programs (plus several others if you need them).
Pidgin doesn't have some of the advanced features of the stand alone products, like voice chat, but if all you do is type chat with people from multiple networks, this program is the one to use. You still need accounts set up through the other IM companies, but once you have the accounts set up, you can use this program for all of your IM contacts.
Pidgin runs on Windows and Linux. Mac OSX users are encouraged to use Adium instead.