Pidgin

If you hate ads, you will find this chat program very useful. I´m not happy with Messenger and all those annoying and blinking advertising messages and every conversation in a separated window.

Pidgin contains no ads, all the conversations are grouped in tabs in a single window, so no annoying blinking background windows any more. Pidgin lets you log in to accounts on multiple chat networks at the same time. You can be chatting on MSN, Google Talk, or Yahoo simultaneously, now you can have all your friends of MSN and Gtalk grouped together. It can run on every Windows version. Pidgin is compatible with almost all chat networks and It can support many more with plugins. Every standard feature today in chat networks is supported, such as file transfers, away messages, buddy icons, custom smilies, and typing notifications. Adding messenger emoticons is easy too.
One little advantage worth to mention in Messenger, is that if you are in a LAN network and you want to use MSN to transfer files, Messenger detects the LAN and transfer speed is drastically increased, Pidgin doesn't support this feature, but it's not a problem if you have a FTP server. Also you can't send print screen images directly like in messenger.