There is something about Web 2.0 logos that we like, their differ considerably from the more traditional corporate logo's of the high street or corporate world.
Those nice people at SimpleSpark have made a video detailing 5000 Web 2.0 logos in 5 minutes. Of note is how many we recognise despite the speed of the movie:
It is also well worth looking at the collage of logos created by Ludwig Gatzke, they make a great print out and under Creative Commons license should be in the book...
If you would like to create your own Web 2.0 style logo, take a look at Hongkiat.com