Does an app have shortcut you don't like? Go to System Preferences > Keyboard > Shortcuts > App Shortcuts and click the plus. As prompted, enter the exact title of menu action you want trigger and the new shortcut. I use the on all my machines to rebind Quit Safari / Chrome / Firefox /Mail to Cmd + Option + Q to avoid killing the application when I fat-finger Cmd + Q instead of Cmd + W.
Others already mentioned:
- three-finger drag is indispensable. Anytime I touch someone else's laptop I turn it on and blow their minds
- hold Cmd in Spotlight to reavel the path containing the selected item, Cmd + Enter opens that folder in Finder.
- readline keys work in basically every text input on the machine. Want to delete a line? C-a C-k. Delete the word preceding the cursor? M-Backspace.
- Cmd + Down opens the selected item in finder, Cmd + Up jumps one level up the folder hierarchy Cmd + Left/Right expand/collapse