There are several hundred workstations scattered about the MIT campus and if you've ever wondered how you would find someone or send a quick message to someone or a group of people without waiting for them to read mail while they are logged in, Zephyr is the answer.
Zephyr provides a means for centralizing users' locations while they are logged in and allows fast message service (on the order of a few seconds) to one user or many users simultaneously.
When you log in, a program called zwgc (zephyr windowgram client) is run. This program registers you with the Zephyr server(s) and remains in the background to handle incoming messages. These messages appear in the upper left of your screen in the form of windows (hence windowgram). Clicking on the window will cause it to disappear. Here are some simple examples:
This says that joeuser has been logged in to m4-082-5 since the indicated time. The :0.0 indicates the display he is using.
Result: A small window with the sender's name and message appeared on his screen.
For more info on Zephyr, see Zephyr on Athena.