Here are a few websites, which shut down in the last 2 years. I really miss the service.
1) www.learn.com
It was the best resource to learn anything from programming to gardening. The experts from all over the world, used to participate. They wrote step by step instructions on different subjects, for eg. "how to fry fish?". We could simply login and read! The website would remember for you how many pages of the tutorial you have read, and let you read the next part when you login next time.
2) www.adcritic.com
The site had recorded clippings of TV Ads on various products.
3) www.askme.com
Thousands of people were available online to answer your questions for free!
4) www.e-quill.com
Visual Markup tool, the instructors, researchers all over the world used and liked it very much.
5) www.eb.com
You will have to pay $10 Per Month to access The complete Encyclopædia Britannica with related Internet sites. This site was "free" for about 2 years!
6) Snarf
was an rss news aggregator from tatochip.com that stuck itself into the search bar of internet explorer.
7) onclave.org
went offline in early 04. A perfect corporate blog tool I will really miss!
8) watchthatpage.com
Just noticed it's offline August 26, 05

Failed Technologies --
http://phpwiki.sourceforge.net/phpwiki/
Edit and save pages on the net.