What un(der)-documented features have you stumbled upon in Mountain Lion?

This question serves to share and collect the enhancements which are not documented by Apple, or documented poorly.

Please justify your answer; if it is something that is well documented by Apple and elsewhere on the web, it does not belong here. Your answer should put the feature into context, explaining how to use it. Add a picture if necessary.

Please add only one topic per answer. Try to find and edit existing features / answers rather than posting a duplicate answer which will end up getting deleted. Note that answers which are not specific to Mountain Lion will be removed as well. You can post such answers here.

You can now silence bouncing application icons in the Dock by just hovering over them, instead of needing to click and activate them.

'Do Not Disturb' Mode for Notification Center

In the Notification Center drawer, scroll up a bit past what appears to be the top. There's a switch 'Show Alerts and Banners' that is checked by default. Uncheck it to disable notifications for the rest of the day.

Alternatively, option-click on the Notification Center menu bar item to activate Do Not Disturb mode quickly.

Exposé for all windows

Mountain Lion re-introduces the pre-Lion feature to be able to disable group by application in Mission Control, this was enabled by default in Lion and could not be disabled.

You can disable this in:
System Preferences → Mission Control → Group windows by application.

Save As

...is back!

Try the following keyboard shortcut: option-shift-command-S. Or simply hit the OPTION key while the file menu is pulled down: