I'm using Psi, it's a very good cross platform Jabber client written with the Qt toolkit. There are many others, check http://www.jabber.org/ and pick the one that suits you best. Jabber has server gateways to other IM services like Yahoo/MSN/AIM/ICQ so you can also contact your friends on these services. Check that jabber.org link for more info or look here for a brief description.
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