"Google SubdomainsFollowing is a list of 40 most popular subdomains of google.com in terms of unique visitors. If traffic is any measure of popularity, this chart definitely provides some good hints about the relative popularity of various Google services."

http://www.labnol.org/internet/popular-google-subdomains/5888/

Google Answers is still getting more visitors than Checkout and Code.

It's according to Compete's data, you don't know how much them.

I tried some of the listed domains with which I was unfamiliar.
I seemed to get either the google web search page or a DNS error. On earlier trials I was also getting a google error page for some. None of these seven produced a special page. Below is a summary. I tried each of these with three different browsers.

09. web search ..... http://clients1.google.com/
11. web search ..... http://suggestqueries.google.com/
17. web search ..... http://ap.google.com/
33. web search ..... http://services.google.com/
36. DNS error ...... http://clients.google.com/
38. DNS error ...... http://l.google.com/
39. web search ..... http://afp.google.com/