WE CHOOSE THE MOON
It's name taken from John F. Kennedy's famous 1962 address to Rice University, this is an interactive history exhibit from NASA, allowing you to experience the Apollo 11 Mission, stage by stage. If you're a space buff, you'll like this.
www.wechoosethemoon.org
HUBBLESITE
Sometimes you need to be reminded that you're just an insignificant mote in an unfathomably large universe. Other times, you just want to see kick-ass images from the most righteous piece of optics we've ever blasted into the cosmos.
www.hubblesite.org
80 GIGAPIXEL LONDON
Who's currently winning the "worlds largest photo" race? Right now, the king is a 360-degree panorama of London. Clocking in at 80 gigapixels (that's 8,000 times the resolution of your average point-and-shoot camera), this panorama is detailed enough to read the license plate on a faraway bus.
http://www.360cities.net/london-photo-en.html
WAYBACK MACHINE
There's no greater tool for exploring Internet history than this. Simply put, the site will take any URL and create a gallery of historical versions of that website for you to explore. Check out your favorites or your own sites, and marvel at how far we've come in just five or ten years.
http://www.archive.org/web/web.php
STAR WARS WEATHER
We've had about enough of weather websites that just tell you boring, useless details like the "temperature" and the "relative humidity". Star Wars Weather cuts through all the BS and tells you what you really want to know: what Star Wars planet the weather outside most closely resembles right now.
www.tomscott.com/weather/starwars
THE F***ING WEATHER
Ok, so Star Wars isn't your thing. In the same vein as whatthef***shouldimakefordinner.com, thef***ingweather.com gives you just the facts about the weather - and doesn't sugarcoat them. Not safe if someone nearby is offended by large, Times New Roman profanity.
www... you'll have to type out the URL yourself
GOOGLE BUILDING MAKER
Google's built a lot of cool tools and toys, but for sheer "oh neat" value, this is great. The Google Building Maker allows you to pick a building from any of dozens of cities and build a 3D model of it. If your model is any good, it'll even get displayed for everyone to see on Google Earth! It's super easy and pretty soon all Google Earth cities will be spectacular.
http://sketchup.google.com/3dwh/buildingmaker.html
BSOD
Load this page on someone's browser, and hit F11 to enter fullscreen mode. Bonus points if they have an unsaved document open when you do it!
www.bluescreenprank.com