1. IRC – enables instant exchange of messages and discussions ("chat")

Links to the most commonly used IRC servers
http://www.irc.org/links.html

Information about the IRC system
http://www.mirc.com/irc.html

2. ICQ (I Seek You) – the most popular instant messaging service

Official ICQ web site
http://web.icq.com/

ICQ2Go – system which enables the web use of ICQ
http://go.icq.com/

3. AOL Instant Messenger – America Online instant messenger

Official AOL Instant Messenger (AIM) web site
http://www.aim.com/

AIM Express – contacts and instant messaging over the web
http://www.aim.com/get_aim/express/aim_expr.adp

4. Yahoo Instant Messenger – instant messenger by the Yahoo

Official Yahoo Instant Messenger web site
http://messenger.yahoo.com/

Information about different possibilities, including video and audio communication
http://help.yahoo.com/help/mesg/index.html

5. MSN Messenger Microsoft instant messaging service

Official MSN Messenger web site
http://messenger.msn.com/

Different communication options of the MSN Messenger
http://messenger.msn.com/Feature/Communicate.aspx?mkt=hr-hr

6. Trillian – instant messaging server compatible with the mentioned services

http://www.trillian.cc/