Yahoo! Messenger - Instant messaging allows you to quickly exchange messages with your online friends. Unlike e-mail, instant messages appear as soon they`re sent. By downloading the Yahoo! Messenger application to your computer, you can send instant messages to other people from around the globe, send them files or have a voice conference.
Other features include voice chat, file transfer, mail and stock quote alerts, sports scores, news, and much more.
i use yahoo because he keeps your offline messages and for logging capabilities. i dont know if msn come with them, at leats 1 year ago msn didnt have this kind of capabilities.
I use Trillian (search for it on Google) and it has Yahoo, AIM, MSN, IRC and ICQ all in one. It keeps logs of all conversations on all messengers. Best of all, it takes up about as much space as only one of these programs, so it's kinda like 5 for 1 (space wise).