ICQ is a (largely) text communication programme and is popular with several 'catters and is even used during the Mudcat Radio (yes, you can actually use it to communicate with Max, Bert, MaryMac and others during the show).

It is available as a free download and versions exist for PC and Mac (I believe they have a Java version too that will run on Linux). It is well worth a look - go to http://www.icq.com.

Jon