Windows

A printable list of common shortcuts can be found below

Switching from program to program

Alt + Tab - Quick cycle through active programs 

Windows key + Tab - See all open windows

Alt (hold) - Keeps focus on all open programs while cycling 

Switching through active programs can be done quickly by hitting Alt Tab, you can also keep focus on all open programs by holding down alt while cycling

Close active window - Ctrl + W

Close active program - Alt + F4

Forces a program to close. If no other program is in focus Alt + F4 will open a shutdown dialog window.

Show desktop - Windows + D

Minimizes all active windows and immediately shows the desktop

Windows search - Windows key

The Windows key can be used to search the OS, simply hit this key and start typing. Programs, documents, settings and even web suggestions will show in the search results. 

Copy screen (Snipping tool)

Windows + Shift + S

Capture the screen currently in focus. This function is part of  the Snipping Tool. Use Windows Search in the start menu to locate the tool.