YouTube - When typing in the search box, recently made searches are shown in purple.
/via Davide Dettori
Google Analytics - To select a period of time, you can select a complete month by clicking the month title
Google Images - When searching for some images which are somehow connected with recent events, it shows a little label with its age
Gmail - Typing ? in Gmail (when not in a input field) gives you a cheat sheet of all available key commands.
Google search - When hovering over a suggestion, you see preview of the image search results for that term.
/via Floris Jansen
Google Calendar - When an event is continuing to the next week, they give you a little arrow.
/via Pierre Valade
Google Maps - When zooming in to the maximum, the zoom in button merges with the street view button indicating that the next zoom level is with street view.
/via Toby Adams
YouTube - When typing in the search box, the autocomplete dropdown size will be limited so it doesn’t cover the video.
Gmail - While in the inbox, the Compose button is red. Then when composing an email, the Sendbutton is red instead.
/via Joe Sparano
Feedburner - You see this message after you set up your feed and want to view your Feed Stats right away.