Can you name Google’s top ten products? If you look at how Quantcast ranks Google’s subdomains, you can get a sense of which Google products are the most popular, since they each have their own subdomain. Google’s main search engine tops the list with an estimated 136.6 million unique visitors in the U.S. Then comes Google Maps (36 million), Image Search (31.7 million), and Gmail (10.5 million). Google Docs, Sites, and Knol are still too small to make the top-ten, but are all showing decent growth.

YouTube, Blogger, and Orkut are not included below because they are on their own domains, but YouTube would be second with 70 million unique U.S. visitors, and Blogger would be in the top 5 with 23.5 million. Orkut is not popular in the U.S., so it would not be a factor in this particular list. And I took out sorry.google.com, the domain Google uses to try to catch bots and spyware. It would have ranked No. 8.

Another notable trend that Quantcast measures is the rise of Google Book Search, which it claims surpassed Google News in visitors back in September. Perhaps the enhanced newspaper archive search features it introduced back then helped. Book Search visitors are now at 8.4 million vs. 7.4 million for Google News, according to Quantcast. Other measurement services such as comScore and Compete show Google News to still be bigger than Google Book Search. ComScore for instance shows Google News at 15.6 million unique U.S. visitors in November, compared to 7.8 million for Book Search. But both also show significant growth in Book Search visitors since the summer.

The Quantcast numbers are not completely reliable because Google’s sites have not been “Quantified” (the process Quantcast uses to collect its most accurate data). But they are roughly accurate. Below the Quantcast rankings, I’ve put the comparable rankings from comScore for U.S. visitors. Some of the rankings are different, but nine out of same products make it on each list. (The only difference is that comScore counts Google Product Search, while Quantcast counts Google Groups).

Quantcast

1. Google Search
2. Google Maps
3. Google Image Search
4. Gmail
5. Google Book Search
6. Google News
7. Google Video Search
8. Picasa
9. Google Earth
10. Google Groups

ComScore

1. Google Search
2. Google Image Search
3. Google Maps
4. Gmail
5. Google News Search
6. Google Video Search
7. Google Product Search
8. Google Book Search
9. Picasa
10. Google Earth