Alvaro’s Messenger

Alvaro’s Messenger (aMSN) is another instant messenger client that is open sourced and supports all the major operating systems :linux, Windows, MacOSX and BSD variants. There is a number of plugins and allows you to sign into more than one account (as do most all-in-one clients). Pretty much all of your usual IM needs are covered in this, including tabbed chat windows, conferencing, groups, file transfers and more. A lot of customization can be done to this.
While it’s not the first open source instant messaging client, it is one that seems to be more well-rounded and has some support behind it.