Well every provider has different access methods, of course.
Google's is probably the easiest since you can easily just supply a username/password (note: I believe they support OAuth as well, which would be the preferred means of accessing sensitive data instead of asking users for their password) and utilize their contacts API.
http://code.google.com/apis/contacts/
Google's is probably the easiest since you can easily just supply a username/password (note: I believe they support OAuth as well, which would be the preferred means of accessing sensitive data instead of asking users for their password) and utilize their contacts API. http://code.google.com/apis/contacts/
When I looked at this several months back, it seemed there had been a few libraries to abstract this at one point, but they eventually gave up. Possibly because the other systems don't/didn't place as nice as google. But there is: http://developer.yahoo.com/addressbook/ and http://msdn.microsoft.com/en-us/library/bb447750.aspx