An ''instant messenger'' system is one that allows people on the Internet to find and communicate with one another in real time (unlike e-mail). 

'''InstantMessenger services and protocols'''
* AolInstantMessenger
* YahooMessenger
* MirabilisIcq
* JabberProject
* WindowsMessenger (and MSN Messenger and .NET Messenger)
* InternetRelayChat (not really a InstantMessenger, but related)

'''The following clients support one or more of the above'''
* Odigo, http://www.odigo.com. It can do ICQ/AOL/AIM, and its own chat protocol.
* CeruleanTrillian, http://trillian.cc (does ICQ/AOL/AIM/Yahoo!/IRC/MSN/Jabber(Pro))
* Gaim (GaimMessenger), http://gaim.sourceforge.net/, is a very good Gnome interface to AIM, as well as other protocols.
* EveryBuddy, http://www.everybuddy.com/ (AIM/ICQ/MSN/Yahoo!/Jabber)
* AYTTM, http://ayttm.sourceforge.net/  (AIM/ICQ/MSN/Yahoo!/Jabber) a fork of EveryBuddy
* iChat, http://www.apple.com/macosx/jaguar/ichat.html.  It does AIM and (apparently) its own LAN-based protocol.

See also InstantMessaging

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Would it be correct to say that the Unix "talk" facility - which (at least in later versions?) could be used across the Internet between compatible Unixen) - was an early form of instant messaging? -- ApoorvaMuralidhara

I think that's close to correct.  Of course ''talk'' was only for the wimps who could not cope with a real Unix tool such as ''write''.  -- DavidVincent