According to hitwise.com:
Hitwise US – Top 20 Websites – September, 2008
Rank Website Market Share
1. http://www.google.com 6%
2. mail.yahoo.com 4.7%
3. http://www.myspace.com 4.01%
4. http://www.yahoo.com 3.88%
5. mail.live.com 2.14%
6. http://www.ebay.com 1.61%
7. search.yahoo.com 1.48%
8. http://www.facebook.com 1.2%
9. http://www.msn.com 1.08%
10. http://www.youtube.com 0.93%
11. http://www.gmail.com 0.79%
12. http://www.wikipedia.org 0.53%
13. images.google.com 0.52%
14. mail.aol.com 0.42%
15. search.msn.com 0.41%
16. news.yahoo.com 0.41%
17. my.yahoo.com 0.4%
18. address.yahoo.com 0.4%
19. http://www.pogo.com 0.37%
20. http://www.craigslist.org 0.33%
(Get all the Info HERE)