IMP, the Internet Mail Program, is a web-based email client that is accessible to Williams people at http://imp.williams.edu. IMP is part of the open-source Horde project.
Tips
It can be annoying to see the oldest messages when you first log in. To change this setting, go to Options -> Display Options, and set it to sort by Arrival Time (Descending).
Don't use IMP on your own computer! It really sucks compared to a nice client like Mozilla Thunderbird. See configuring an email client
Trivia
The first version of IMP was written by a student at Williams, Chuck Hagenbuch '00, and initially deployed at http://webmail.williams.edu/ on WSO's servers. WSO handed off the webmail address to OIT in Spring 2003.