Reposted from ResourceShelf. Shows ranking, reach %, # of unique visitors and page views. Updated monthly on the Google Ad planning site. Lists below from April 2010.

The Top 10
1. Facebook
2. Yahoo
3. Live.com
4. Wikipedia.org
5. MSN.com
6. Microsoft
7. Blogspot (owned by Google)
8. Baidu.com
9. QQ.com
10. Mozilla.com

Here are a few other rankings:

18. Twitter
22. Amazon.com
31. Flickr.com
44. IMDB
56. Linkedin.com
83. NY Times
102. Scribd
166. Reference.com
179. Reuters.com