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.
IMP is a web-based alternative to e-mail clients like Mozilla Thunderbird (for more on these, see configuring an email client). There's no download or installation of software necessary, and it can be accessed through any computer with internet access and a web browser.
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).
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.