Mac OS X Mountain Lion Spotlight - Finds Calendar & Contacts even when you type in their old names (iCal & Address Book) from previous versions of Mac OS X.
iCal - Shows birthdays from Address Book with the person’s name and age.
/via Clemens Mayer
Google Calendar - If you hide several calendars in the list, options menu for the next calendar will default to hide when you expand it.
/via Marina Janeiko
Doodle - Compresses dates that you did not select times in like an accordion. Clicking on the accordion expands the entire calendar.
Google Calendar - When an event is continuing to the next week, they give you a little arrow.
/via Pierre Valade
Google Calendar - Favicon displays the day of the month.