Pidgin (previously Gaim) is a GTK2 based instant messenger application, it’s widely used and supports a wide array of IM services. This works as a great replacement for closed-source IM clients.
Pidgin versions are available for all major operating systems, including Windows, Linux, FreeBSD and Unix variants that run X. On OSX however it is recommended to use Adium, as this is written to run natively on OSX and uses the same core engine, libpurple (previously named libgaim.)