MSN

Stands For
Microsoft Network

Which means
MSN was a popular web portal created by Microsoft that provided many services such as web based e-mail (Hotmail) and an IM client, MSN Messenger, which was commonly known as just "MSN". The service has now been renamed Windows Live, but many users still refer to it as MSN.

Example
Cindy: Add me on MSN Messenger tonight!

Related Acronyms

 * MS
 * IM