Contextual Gadgets greatly accelerate user adoption by bringing the application directly to the user in context and in their normal work flow. Many great apps fail to gain user traction because they are separated from the normal workflow. Gmail Contextual Gadgets solve that problem for developers, and make it much easier for the end users to see the benefits.
Here's an example gadget from Harvest-- allowing the users of their time tracking application to fill out their time sheets directly from within Gmail:
We just posted more info on both our developer-facing and customer-facing blogs. Check it out:
Google Apps Developer Blog