A very useful toolbar (for Internet Explorer / Mozilla / Netscape) that allows you to find websites that other users recommend. It is community based so you can also recommend sites as well. Download from http://www.stumbleupon.com/
Has two ratings - Not for me and I like it. Use on any sites you like (or dislike). The more sites you rate, the better results you get. Sites are put into interest categories (i.e. Web Developer, Programming) - so if you add your interests, you will get better results. A good way of browsing the web without using a search engine.
Almost 300,000 sites listed and 83,000 members. Going since mid-2002 (site redesign January 2003 - approx 20,000 members). Hopefully the more users and the more websites, the better it will get.
Update - Jan 9 2004: 90,614 members, 309,229 websites - seems to be growing quickly, perhaps 10,000+ members by February
Update - Mar 5 2004: 104,872 members, 407,710 websites. Perhaps there will be 1/2 million sites by late April / early May? More member features have been added (member groups - for those with similar interests)