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.