If you want to see a lot of web 2.0 company logos, http://www.go2web20.net/ is a good place to start. It's not designed as a logo library, but it ends up being very good in that respect. It's a little hard to navigate, but boy are there a lot of logo, and they are a good size such that you can see them clearly yet see a lot at once.

Another good site (which is designed to show logos) is http://web2logo.com/ They organize them by category so it's easier to see a particular type, but it's not quite as easy to see them in bulk.