ModalPopupExtender Showing/Hiding
The modal popup extender is pretty cool; it pops up a modal form on the screen with whatever you want to see, prompting the user to perform some action. In previous iterations, you had to use a trick to show/hide the form though, by calling the javascript method rendered for it directly. With the RTM version, there is an easier way. The extender has Show() and Hide() methods respectively to perform the action. It's best to put the code that triggers this in an UpdatePanel, so the page isn't posted back everytime to perform these actions.
I found this very useful to show a details view about the details of a form, instead of putting it at the bottom or right, because I had so much data being rendered on the screen. It worked pretty nicely.