Comscore has released the data for the most widely used search engines . Quoting the release:
Google Sites led the U.S. core search engine market in July 2008 with 61.9 percent of the searches conducted, up from 61.5 percent in June, followed by Yahoo! Sites (20.5 percent), Microsoft Sites (8.9 percent), Ask Network (4.5 percent), and AOL LLC (4.2 percent).
Americans conducted 11.8 billion searches at the core search engines, representing a 2-percent gain versus June. Google Sites handled nearly 7.3 billion core searches (up 2 percent), followed by …

Yandex is the largest search engine database in Russia. Reports point that it could be the most popular search engine used in Russia with about 44% of the Russian search engine market share.
Much like Google has become a standard word in the English dictionary meaning "search", Russians are used to "Yandex" in a much similar matter.
In fact, comScore reports that Yandex is the Number 2 search engine in Europe. Adding to the competition to Google, Yandex will go for a Nasdaq listing in the fall of 2008.

Nielsen has just published the Search Engine market share data through March 2008 for all US search providers. Now we have all the four major firm’s data on who owns the Search Engine market - from Hitwise, Compete, ComScore & Nielsen.

Hitwise
Compete
ComScore
Nielsen
Avg*

Google
67.3
69.4
59.2
58.7
63.6

Yahoo
20.3
14.8
21.3
18.1
19.9

MSN
6.7
10.2
9.4
12.0
10.5

AOL
–
1.5
4.8
4.1
4.5

Ask
4.1
3.7
4.7
2.4
4.2

The trend of search engines usage pattern has changes significantly in the past couple of years. While AOL and Yahoo were the strongest (and probably the most popular ones in early 2000), the new report from Compete shows that Google and Microsoft Live are emerging as the new leaders of today.