These are links that don't deserve their own category (yet) but that I find to be useful:
The Straight Dope -- Cecil Adams answers all of your tough questions, or, failing that, makes a little fun of you for asking them.
Register your birthday with them and/or look up who else has birthdays on any given day. They can also remind you about a friend's birthday via e-mail.
Advice For Undergraduates Considering Graduate School
I was trying to decide whether to go to graduate school or not, where to go, how to go about it, etc. This web page is a useful source of information on the subject.
Want to know what SONAR or ARPANET stand for? Then go here.
Bartlett's Familiar Quotations
A nice resource for finding famous quotations.
Roget's Internet Thesaurus
Want to find out that a synonym for "opening" is "aperture"? If so, this is the place for you.
Ye Olde English Sayings
Tells the origin of many popular phrases such as "Black Market" and "Getting Tanked".
Latin Grammar Aid
If you are studying Latin, then this probably could be of use to you.
The Internet Movie Database
A great source for information about any film, including upcoming releases.
Guide to Lockpicking
Originally known as the MIT Lockpicking Guide, it was disowned by the MIT community when it became publicly available. Nevertheless it is a very useful guide on the subject.
Best Viewed With Any Browser
A page opposed to the "Best viewed with Netscape" and "Best viewed with IE 4.0" tags that are sported by many sites.
Edgar Allen Poe
Includes his complete works and biographical information.
An online library that includes the works of Lewis Caroll, William Blake, Walt Whitman, and more.
Good Quotations by Famous People
The premier child pornography site on the web. Er, I mean, this is a collection of quotes. Really.
Logic & Fallacies
Published by the Atheism Web, this page describes common fallacies in logical argument.
Nick's Guide to Web Counters
Great place to find a web counter, if you are looking for one.
A great place to find great quotations.
A nice online auction service... Lots of great stuff.
An online translator that translates to and from English, Portuguese, Spanish, French, German and Italian. You can type in text to be translated, or give it the URL of a web page to be translated.