I have been using the cross platform multi-protocol open source IM ,Gaim
for over 6 months now. It served every bit of my purpose and I was really satisfied with it. Very low on memory great supprt for protocols (though files transfers are little buggy , but who uses yahoo to transfer files heh ? ). The only problem was the drab GTK+ environment. I did a google search for some themes and i hit this website where i downloaded a utility called gtk2 themes and theme switcher. And also with gaim plugin pack (it has almost every plugin you need) i confugured gaim now to be pretty ! So now I have the best IM .
Here are some screenshots :
To download gaim , visit gaim homepage
sweet