Windows Live Web Messenger

Windows Live Web Messenger is the discontinued browser-based version of Windows Live Messenger developed by Microsoft which allowed users to send instant messages online and in real-time with others using the Microsoft Messenger service from within a web browser. The service allowed users without administrative privileges on their computer, such as at a shared public computer, to chat with others on their Messenger contact list without having to install the Windows Live Messenger client.

Source: Wikipedia — Windows Live Web Messenger (CC BY-SA 4.0)