Greasemonkey has changed the way a web-page is rendered and shown at the user end.
A number of applications have developed using it.
Here is a plug-in of to-do list for google calendar:
Its good only that it is annoying in the sense that it requires you to sign-up.
That's a huge amount of discomfort making it unusable.
Here is another plug-in for adding another inbox to your gmail:
Something which your label in gmail can easily do. Just that it adds a button on the top making it bit easier!!!