Ever wondered about the difference in the methodologies? This explains why there are ranges among the various measurements.

Comscore:
http://www.comscore.com/blog/2007/10/the_challenges_of_reconciling.html

Compete:
http://blog.guykawasaki.com/2007/10/ten-questions-2.html

Nielsen:
http://www.netratings.com/press.jsp?section=pr_faq&nav=7

Hitwise:
http://hitwise.com/other/isp-fact-sheet.php

http://home.compete.com.edgesuite.net/site_media/upl/img/JC-SearchShareNOV1.1.gif
http://home.compete.com.edgesuite.net/site_media/upl/img/JC-SearchShareNOV2.1.gif

COMPETE: Google market share increased to a record 69% of search queries in November on volume gains of 2.5%. That puts market share nearly 10ppts higher with close to 2 billion more queries than the same month last year. Meanwhile MSN/Live held steady, slightly above year ago market share levels. Ask also held steady month-over-month but remained slightly below last November’s market share levels. Yahoo! managed to capture the distinct honor of being the only one of the top players with lower year-over-year query volumes. In fact web search queries on Yahoo! were down 10% from November 2006 despite the huge market gains. The continued losses pushed Yahoo! market share to a record low in November.

http://battellemedia.com/archives/004176.php
http://battellemedia.com/images/2145027894_0457f8bcfd-tm.jpg

Here is an overview comparing Google, Yahoo & MSN's performance over a five month period from June to Novemeber

Comparing the data from Compete, Neison, Comscore, Hitwise

There are more graphs at http://searchengineland.com/071228-173523.php including ones for Yahoo! and Live.