Windows Messenger

Windows Messenger is an instant messaging client from Microsoft that can communicate both via SIP and .NET Messenger Service.

Windows Messenger should not be confused with MSN Messenger (which is limited to .NET Messenger Service), although it looks very similar.

It has a variety of features: instant messaging, presence, file transfer, application sharing, whiteboarding. However, its most notorious feature was the security hole that allowed people to send unwanted adware through it.