In my never ending journey to find the infamous perfect instant messenging client, I downloaded Psi, a client for the Jabber initiative. With Jabber, you can connect to other IM networks (ICQ, AOL, Yahoo!, MSN, etc.) and communicate with those users easily (at least, that's how it's supposed to work).
This system is still young and blossoming. Unlike other IM networks, Jabber has different servers that you can connect to with different capabilities (specifically, not all servers support communication with all other IM networks). The server you join becomes part of your username and cannot be easily switched, so be careful.
Like any young system, Jabber isn't without its problems. I've yet to find a way to simultaneously use my ICQ and two AOL IM accounts and importing contact lists is messy at best. However, Psi enables me to get in touch with the Jabber and ICQ users I needed to reach, so it does exactly what it's supposed to in that respect.
Given time, I think Jabber has what it takes to be the best IM network. However, like most open source software I've used, it seems to take forever for that to happen.
