Skype - Use your computer to talk to computer users on Internet, or use your computer to call them on their landline telephone. Computer-to-Computer calls are free and calls to telephones are very cheap (16c per minute internationally from South Africa). Currently one of the most well known, now with instant text chat as well.
MSN Messenger - free computer to computer voice calls and text messaging. It will also video conference, share whiteboards and applications etc. It will notify you the moment a contact of yours is logged onto the Internet. If you have trouble working through a firewall try http://webmessenger.msn.com/.
MSN Plus! - if you use MSN Messenger you WILL want this free add-on that helps organize contacts, give custom away messages, auto reply messages, and a host of other customizable features for Messenger.
Pidgin - Pidgin (previously called Gaim) is a free multi-protocol instant messaging (IM) client for Linux, BSD, MacOS X, and Windows. It is compatible with AIM and ICQ (Oscar protocol), MSN Messenger, Yahoo!, IRC, Jabber, Gadu-Gadu, SILC, GroupWise Messenger, and Zephyr networks. There is also a work around to get it to work with Googe Talk.
Meebo - browser based access to AIM, Google Talk, MSN Messenger, ICQ, Jabber and Yahoo! Talk. Great for bypassing firewall blocks to these popular IM 's.
GTalk-to-VoIP Gateway - free and publicly open voice gateway for major Instant Messenger clients. It makes possible voice interoperability between Google Talk, MSN/Live Messenger, Yahoo! Messenger and SIP phones without any additional software installation. Originally, the gateway was developed with Google Talk support only (and made calls to PSTN), from which it derived its name.
Hellkom - Giving our monopolistic Telco a tough time. Has some interesting views as well as alternative ISP connection options in South Africa.