popuri.us
A tool to check at-a-glance the link popularity of any site based on its ranking (Google PageRank, Alexa Rank, Technorati etc.), social bookmarks (del.icio.us, etc), subscribers (Bloglines, etc) and more!
In-Depth Look Inside the Twitter World
Sysomos Inc. conducted an extensive study to document Twitter's growth and how people are using it. Interesting.