Jeff Bonforte never met an API he didn’t like. The CEO of Xobni, a startup that makes an outlook plug-in that makes your e-mail smarter, has been busy getting his team of engineers to integrate every possible API they can think of into the service. Xobni already added LinkedIn last June. Today it is adding integrations with Yahoo Mail, Facebook, Skype, and Hoovers. Data from all of these services appears in the Xobni sidebar in Outlook. Let’s take them one by one.

Xobni has been working on its Yahoo Mail integration since last April. Now in the sidebar, users can search their Yahoo Mail messages and see contacts and attachments. To send or receive email through your Yahoo account, however, you still have to click through to Yahoo Mail in your browser, which is sub-optimal.

Every time someone sends you an email who is also a Facebook member, you can see in the sidebar their current Facebook status message, general profile information, their Facebook picture, and recent updates made to their profile.

The Skype feature lets you send instant messages, SMS messages, and make Skype or regular calls to contacts who are also Skype users. Again, as with Yahoo Mail, actually making a call or sending an IM automatically launches the Skype application.

Finally, the Hoover API brings up Hoover company information for each contact. That provides some helpful context when responding to business emails, although I find the LinkedIn data more useful.