Pidgin

The instant messaging client that rules in Linux, previously known as Gaim, also convinces after crossing over to Windows. It includes all the possibilities that it already had, that are: an extensive list of instant messaging nets to connect to, capability to be modified with plugins and extensions, allowing to start sessions with vairous accounts at the same time,...

  Pidgin is an instant messaging software with quite a lot of popularity, specially because it is capable of connecting to all the nets: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr.

  Like any other client, it supports file sharing, emoticons and all it is completely customizable, but this isn't all, the conversations are organized in different tabs, is saves the conversations, it lets the user know about any incident, with both the usual window and sound effect,...

  To resume, one of the best, most reputed and secure instant messaging clients.