Pidgin is a freeware chat client that lets you login to multiple chat accounts at the same time. Pidgin is an open source program so you can modify it by making changes in the code.
Pidgin runs on Windows, Linux and Unix Operating Systems.But for Mac Os X you need to download the plug-in.

Pidgin is compatible with the following chat networks : AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. It can support many more with plugins.

Pidgin supports many features supported by the networks such as file transfer, buddy list, buddy icon, custom smilies, unread messages and custom notifications. More features can be added by installing the required plug-ins.

Pidgin can be translated into many languages, so you can chat with your friend in your native language. Pidgin is truly customisable. You can select the smiley themes, add new smiley themes, customise the sounds when you log in or log out etc. You can use proxy server if you want and also you can  manually specify the range of ports to listen on.

So Pidgin is a user friendly IM that lets you connect to your friends in different networks in a very easy and simple way.

Download Pidgin here