While there is a dearth of "official" IM clients for the Linux desktop, application such as Kopete more than make up for it. Kopete aims to be an instant messaging solution which can connect with nearly every popular network, and as such comes with inbuilt support for major protocols such as those of Yahoo! Messenger, AIM, Windows Live, ICQ, and XMPP (Google talk), and more. (Although the Windows version doesn't seem to support XMPP (Google Talk))
Kopete has a compact and light UI, yet packs some powerful features. With support for identities you can create multiple "packages" of accounts for different people. Or perhaps a work and a home identity which have different configured accounts. This makes handling multiple IM accounts particularly easy.
The application is highly configurable, allowing you to configure every aspect of the programs looks, and behaviour. With support for plugins, you can add features not present in the application. The bundle of plugins which comes with KDE4 allows for some advanced IM features such as automatically bookmarking links received in chats, keeping a history of chats, automatically replacing text, translating chat messages, previewing image, piping chat messages through an external script, etc.
Kopete is indeed a powerful IM with many features which you will be hard pressed to find in other Windows or Linux IM applications. However for now, it is of limited use in a Windows environment due to the lack of XMPP, which is required for the popular Google Talk network.