Our API directory now includes 37 campaign APIs. The newest is the Experian CheetahMail API. The most popular, in terms of mashups, is the LaunchBit API. We list 1 LaunchBit mashup. Below you'll find some more stats from the directory, including the entire list of campaign APIs.
Additional third party integration with Gmail has been possible (you only have to look at all the contact import features from sites like Facebook), but these have either been clunky manual procedures that involve exporting contacts to a CVS file, or they require you to supply your email password. Neither option is all that attractive. Now there is an alternative. Google have recently added the ability to access the IMAP/SMTP features of particular Gmail account using OAuth.