Google tends to work on some pretty interesting projects from time to time. If you’re using Gmail, then you can turn some of these on to experiment.
They’re called Gmail Labs. To access them, head to the Gmail Settings page, and then click on “labs” at the top of the screen.
There, you’ll be able to pick from a bunch of different options including widgets and apps. They’re experimental, so they may not all work properly, but you may find some that you like and want to use frequently.