Mac pro tip: Option-click everything. It's remarkable how many secrets are lurking behind near every UI element.

And once you've option-clicked, also option-shift-click, command-click, control-click, etc.

An easy favourite is the Wifi menu bar item. Option-click to see all sorts of useful info about your current connection, including IP address, MAC address, connection speed (Tx Rate), the amount of noise, etc. And when you've option-clicked the menu, hovering over network names shows you the key info about that network.

I don't get why Apple just doesn't provide more UI tips, like tooltips on first use of the UI. There are so many little things like this that would be great to know. For example on Windows Dialog popups, the labels for each option underline the letter for the keyboard shortcut. Did you know that Apple has keyboard shortcuts for dialog popups too, if you press CMD+letter? It took me years to figure that one out.