Instant Messenger
Instant Messenger are very popular. You have to register your nickname at one of the companies and download their software. Then when you are in the Internet you can start the software and log onto their servers. Since then you are marked as "online" and all your friends who know your nickname and get the same Instant Messenger can see that you are online and easily chat with you. Every of the 4 big players has its own software client which contain advertisement, spyware and is not compatible with other IM protocols. I recommend you to download Miranda, which is a open source Instant Messenger which is very small, without ads or spyware and working without installation. It works great with every IM protocol, even at the same time. http://miranda-im.org/. You can also test a WAP-ICQ/MSN Gateway: http://www.tjat.com/ and http://www.wapd.de/ for example.
More info: http://www.encyclopedia-online.info/Instant_messenger
http://www.infoanarchy.org/wiki/wiki.pl?Instant_Messenger
http://nscsysop.hypermart.net/no_chat.html
ICQ
Users: 7 million
Login server: login.icq.com or login.oscar.aol.com
Used ports: tcp at any port you choose in the settings (default is 5190)
Used protocol: Oscar
Supports proxy: http, https, socks 4 and socks 5
Online version: http://go.icq.com/ (connects to iht-d01.icq.com at any port you choose, default is 80) or http://www.odigo.org/features/express.html (The Odigo client online, supports all 4 services)
More info: http://www.encyclopedia-online.info/ICQ
http://www.rejetto.com/icq/
MSN Messenger
Users: 23 million
Login server: messenger.hotmail.com
Used ports: tcp at 1863 which you can not change, but if connection failes, MSN tries port 80. (voice/video/webcam is tcp 13324 and 13325, application sharing/whiteboard is tcp 1503 and file transfer tcp 6891)
Used protocol: .NET Messenger Service
Supports proxy: http, socks 4 and socks 5
Online version: none official, but several inofficial. Be careful to give them your password! http://www.odigo.org/features/express.html (The Odigo client online, supports all 4 services), http://messenger.lycos.co.uk/messenger/ (Lycos Messenger, works also with Yahoo) and maybe you can try http://www.mister-i.com/i-mode/messenger.jsp (i-mode, after 3 days it costs $) or http://kickme.to/msnmessenger2go (he's still working on it).
More info: http://www.encyclopedia-online.info/.NET_Messenger_Service
AIM
Users: 60 million
Login server: login.oscar.aol.com, toc.oscar.aol.com and login.icq.com
Used ports: tcp at any port you choose in the settings (default is 5190), for the IM images the software uses port 4443 (?)
Used protocol: Oscar
Supports proxy: http, https, socks 4 and socks 5
Online Version: http://toc.oscar.aol.com/ (The old QuickBuddy, port 80) and http://toc.oscar.aol.com/aimexpress/index.html (the newer AIM Express, port 80) or http://www.odigo.org/features/express.html (The Odigo client online, supports all 4 services)
More info: http://www.encyclopedia-online.info/AOL_Instant_Messenger
Yahoo Messenger
Users: 20 million
Login server: cs.yahoo.com or cs.yahoo.co.jp (maybe different)
Used ports: tcp 5050 and 80 for file transfer, which can be changed in the settings
Used protocol:
Supports proxy: http, socks 4 and socks 5
Online Version: http://messenger.yahoo.com/ (The official Web Messenger) or http://messenger.lycos.co.uk/messenger/ (Lycos Messenger, which also supports MSN) or http://www.odigo.org/features/express.html (The Odigo client online, supports all 4 services)
http://www.ksc-creative.com/ytp/Wiki.jsp?page=YahooTranslatingProxy
More info: http://www.encyclopedia-online.info/Yahoo!_Messenger