Box.net - Free online file storage and sharing. There is also a paid service as well, but a free account will give you 1GB of online storage.
Google Docs (Writley) - Free web-based word processor and spreadsheet, which allows you share and collaborate online. Why pay for Microsoft Office, when this is free?
Del.icio.us - Social Bookmarking tool. Let’s face it, in college you are going to be using more than one computer. Here your bookmarks are organized by your tag, all in one place.
Meebo - Allows you to access Instant Messengers from anywhere. Meebo is an AJAX-based chat client where you can use AIM, Google Talk, Yahoo, MSN Messenger, Jabber, ICQ, and more. No download is needed, so it's convenient when using different computers.
Wikipedia - free online encyclopedia. Though it has been under debate as of late on whether it can be used as a valid reference in research. Many professors have banned students from using Wikipedia as a cited resource. Still, Wikipedia is a good tool for finding information quickly.
Stu.dicio.us - Student organizer and social note taking tool where students can manage schedules, grades, to-do lists, collaborative notes and files. Also see MyNoteIt.
BackPack - Backpack is an all around great organizer including note taking, file storage, to-do lists, a calendar, and more.
Google Language Translator: You can use this tool as a proxy, which means you can use it to access restricted sites on campus, in libraries, etc. To use the Google Proxy, just enter the url after: http://www.google.com/translate?langpair=en|en&u={your url here}
Online Payments: PayPal and Google Checkout - Pay fellow classmates and roommates easily.
Selling back textbooks can be a great way to recoup some of the costs of the outrageous prices of textbooks. Unfortunately school bookstores usually...