Pidgin is an tiny application that allows you to connect with all your friends through most of the chat networks in one place. It supports AIM, MSN, Yahoo, Google Talk, IRC, QQ, Sametime, Zephyr, Bonjour, Groupwise, SILC, XMPP, Gadu-Gadu, ICQ, MySpaceIM, SIMPLE (upphhh - that's a lot), and it can support many more with plugins. Pidgin runs on Windows & Linux. If you want an chat aggregator app for Mac download Adium.
From pidgin.im:
Pidgin supports many features of these chat networks, such as file transfers, away messages, buddy icons, custom smilies, and typing notifications. Numerous plugins also extend Pidgin's functionality above and beyond the standard features.
Pidgin integrates with the system tray on Windows, GNOME2, and KDE 3.1.
Pidgin is under constant development. Releases are usually frequent and driven by user contributions, such as bug reports and patches.
Pidgin is free and contains no ads. All our code is open source and licensed under the GNU General Public License. This means you can get Pidgin's underlying code and modify it to suit your needs, as long as you publish the changes you make for everyone to benefit from as well.
