We live in a cross-platform world. People work in front of their Windows PCs all day long, then go home to their Mac. Or they code at their Linux terminal then unwind with games on their Windows box. Unfortunately, for as many cross-platform people as there are, it doesn't always seem like there's a lot of software built to follow them from machine to machine. Much of the time, even when a piece of software is available for several operating systems, it doesn't ever work quite the same. Configuration options are different, keystrokes behave differently, and nothing looks quite right.
One exception is Pidgin, the Linux- and Windows-compatible, multi-network IM client built on libpurple, the open source library that started life as "gAIM," an AIM client for Linux users.
Pidgin is a deceptively simple piece of software that presents a no-frills face to new users, but offers a lot of tweakability once they're comfortable with it. It's cross-platform thanks to a Windows port of the GTK toolkit, a programming library originally written for the popular image editing program the GIMP. GTK has since gone on to drive quite a few open source and free software projects and forms one of the foundations of the GNOME desktop.