Entertaining

Blue Ball Machine (blueballfixed.ytmnd.com)
Work safe, it's a large collection of blue balls running through a factory. Perfect for the A.D.D. child next door, it keeps me entertained for days.

The Onion (www.theonion.com)
Read it. Just don't take it too seriously. (Also note that this may sometimes not be as safe for work as the other links.)

Technical and/or Nerdy:

Digg (www.digg.com)
Tech geek links that are voted on by the community and the best ones see the front page. Not the fastest news, but definitely interesting.

Dolphin Project (dolphin.twistification.net)
A web site that keeps track of your keystrokes. Very amusing. I have quite a few.

Download Squad (www.downloadsquad.com)
Various new and geeky downloads.

Engadget (www.engadget.com)
Various new and geeky electronics.

Everything2 (www.everything2.com)
Random stuff about, well... everything.

Slashdot (slashdot.org)
News for Nerds. Stuff that matters.

Think Geek (www.thinkgeek.com)
For all products geek-like.

Sysinternals (www.sysinternals.com)
Some of the coolest Windows utilities around. Some of them are old, but still work great!

Webfroot (www.webfroot.co.nz)
Several geeks involved in web development and design writing about what they find on the net.

Wikipedia (en.wikipedia.org/wiki)
The greatest encyclopedia ever. Period.

Wired Magazine (www.wired.com)
Again, I don't visit this one a lot but people say it's cool. It's a little too trendy I think. Still interesting though!