These are some sites, pulled from my bookmarks, that I use frequently, find interesting, or enjoy. This page is a bit of a stub; it could certainly have many other links on it.
These websites have an obvious practical use.
Online Stopwatch: A quick way to set a timer when you don't have one handy.
PDF My URL: This is an easy way to archive a website or create a version to print or save.
T1 Shopper Online Tools: This is a useful site that can perform some of the most common Internet diagnostic tools such as ping and NSLookup. You may not know what these are right now, but if you ever need them, here they are.
Unofficial Google Advanced Search: A nice documentation of loads of Google features that basically nobody knows about. See also my Tips for Using Google.
Wolfram | Alpha: Google is a great way to look for websites. But what if you just want a statistic or the answer to a simple question? Because it uses its own database instead of searching external sources, it is at once more useful and less versatile. It's certainly worth a bit of time experimenting, and is extremely powerful; not only can it look up simple information, it can also compare two similar things (just type what you want to do right into the search box). And if you're missing your calculator, you can do almost anything you could on a good graphing calculator, and have all sorts of other useful information calculated automatically.
These are more in the "waste of time" category, though by no means applicable only if you're actually bored.
ASCII Generator: A cool way to create lettering out of characters like / | =. Makes a unique email signature or something similar.
Computer Stupidities: A collection of silly mistakes made by computer users of all levels. From "I put 'x=x' in my program code so that x doesn't lose its value" to "My computer is smoking and shooting sparks. Should I turn it off?" to "Do I have to be online to back up online?", this collection quickly makes you wonder how the human race can have such a departure of basic logic, besides giving you a good chance to laugh at the mistakes that people new to anything will inevitably make.
Not Always Right: A collection of anecdotes from misinformed and stupid customers everywhere.
Things People Said: A collection of funny and misspoken quotes.
UserFriendly: A geeky webcomic, one of the first popular ones.
xkcd: Another webcomic, probably one of the most frequently linked sites, due to extremely funny takes on everyday occurrences.