Best of The Web Lists.

The Observer issued it's list of Websites that changed the world, Time Magazine produced its list of the 50 "Coolest websites"

Of course lists like this reflect the opinions of the journalists who write them. For an insight into what web users think there are a number of tabulations of the most important/influential blogs based on factors like traffic, number of citations on other blogs and number of links. Blogstreet produces one such list NZ Bear's blog ecosystem is a playful take on the same sort of thing. For a more instant snap-shot Bloglines and Technorati both rank web posts by popularity.