Web 2.0 - the LONG tail!
Although it appears to have been up a couple of weeks, I just came across the Web 2.0 traffic list sponsored by Seth Godin on Alexaholic here.
There are almost 1000 different Web 2.0 sites listed in the traffic index - they range from commercial sites like MySpace to collaborative web sites such as BitterCyclist (an effort to chart bike accidents in San Francisco) based on community effort.
Well worth a look if you are interested in what's hot (and what's not!) in the Web 2.0 space.