For several years now use Meebo, a free web service that allows you to connect to the various IM (MSN, gTalk, Jabber, ICQ, etc..) via web interface.
Meebo is fully developed in AJAX, and the team of developers is constantly updated with regard to the backend. The developments, some short goods and tips are listed in the official blog of Meebo. As regards the connection to the IM user Unregistered will have access to any IM available, but not all simultaneously, among other things will not change the custom avatar. The user recorded can instead connect to different clients with a single password, change your avatar (which is also stored in subsequent connections) changing skin.
Meebo is used all over the world, and leans to the libraries "libpurple”, which also uses the same Pidgin, the well-known IM client included in the major GNU / Linux distributions.