1. Remember how to check a dictionary listing for a word in Safari on a Mac? Hover the cursor over the word and press ⌃⌘D. If you aren’t familiar with those key symbols, here’s a reference.
2. To open old or outsider applications on GateKeeper protected Mac systems (Lion and newer), right-click the app and choose “Open” from the resulting contextual menu, then when asked to confirm that you want to open the app, click the Open button. It might be helpful to view the Robservatory blog post that tells you how you can “jump through some hoops”
3. If you put a folder in the Dock, by default it becomes a stack. If the folder has an icon (iCon? :)), you can cause that icon for the folder to be displayed in the Dock by right-clicking the stack in the Dock, then selecting Folder rather than Stack in the “Display as” area of the contextual menu.
4. If you want to open a Docked folder in Finder, a fast way is to click the folder’s Dock icon, press the up arrow, then press return or enter. I find it easier to press those two keys in sequence than to find and click the arrow icon in the contextual menu that appears when you click the folder’s Dock icon, especially if there are a lot of items in the folder.
5. For most any window containing a search field, ⌥⌘F will put the cursor inside it. (If you aren’t familiar with those key symbols, here’s a reference.)
6. You may know that you can press and hold ⌃ (control) then press the ⏏ (eject key) to bring up the Restart/Shut down dialog on your Mac. Despite conventional Human Interface Guidelines, which would seem to indicate that clicking a Cancel button in a dialog box reverts any changes made up to that point using that dialog box, you can change the status of the Reopen windows when logging back in check box even if you select Cancel after the change has been made.
7. To view the total disk space occupied by more than one Finder file system object, select multiple items in Finder, then press ⌥⌘-I to reveal the Multiple Item Info window.
8. You probably know you can hold down ⌥ (option) immediately after you hear the startup chime to select a startup volume for the current session (if more than one is available) by clicking ꜛ (Up Arrow) beneath the volume’s icon in the row of icons, However, if you hold ⌃ (control) after the startup volumes appear, ꜛ (Up Arrow) will become ↻ (Circle Arrow), and the startup volume you choose will remain selected as your startup volume until you make a different selection. This might save you some trips to the Startup Disk System Prefpane.
9. Select a bunch of images in Finder (or press command-a if your active Finder window displays only images), then press command-option-y to start a slide show.