Ubiquity seems pretty neat:

The great power of Ubiquity—from a developer standpoint—is how easy it is to create commands. With only a couple of lines of Javascript, Ubiquity enables even casual web developers to drastically enhance the features of the browser. From an 8-line command to insert a contact's email address in any text field, to a 50-line Twitter integration, this tutorial walks you through the process of being generative with Ubiquity.