System wide / general

Fn + ArrowDown / ArrowUp Page down and page up
Shift + Control + Eject Lock the system
Command + Shift + 3 Screenshot of entire screen, add Control key to save it to clipboard
Command + Shift + 4 Select part of screen to take screenshot from, add Control key to save it to clipboard

In Browser

Command + L Highlight location bar
Command + T Open new tab
Command +, – Zoom in and zoom out
Command + 1,2,…9 Change the current tab

In Finder

Command + Shift + A Goto applications folder
Command + Shift + D Goto desktop folder
Command + 1, 2, 3 or 4 Change finder view
Command + i Get file info
Command + C Place file in buffer
Command + V Copy the file
Command + Option V Move the file