If you use Google mail then you have probably seen that it is currently trying to sell something called Google Buzz
Unless you specifically disable it (a link at the bottom of the inbox page), the default user profile it produces for you (and which is accessible by anyone who looks) includes a list of all the people you email a lot!
http://www.businessinsider.com/warning-google-buzz-has-a-huge-privacy-flaw-2010-2
Later See comments - it looks like the profile doesn't actually appear until you post to Buzz for the first time - still a VERY stupid way to do things!
And
Much later And another pointed out by