Mandrill, deliver incoming email to a webhook, and process outgoing email. Can do message manipulation also. 12k emails/month free, from MailChimp.
An API framework for CakePHP. They've implemented the Twitter API, and a handful of others. Very cakey, fits into the whole CakePHP approach very well. Not sure if it'll actually be that useful or not.
CloudMade are a mapping technology company built by some of the people behind OpenStreetMap.org, using that data as a base. Professional open source mapping services.
This site allows you to use Amazon's Mechanical Turk API to create HITs via their web interface. It is more powerful than Amazon's own interface because it uses the API. Small accounts are free, higher volume accounts cost money.