The following shortcuts can be used throughout Windows and Linux applications to enhance your productivity (I can see some people still didn't adopt those):

+C - Copy
Ctrl+V - Paste
Ctrl+X - Cut
Ctrl+Z - Undo
Ctrl+Y - Redo
Ctrl+A - Select All
Ctrl+N - New document
Ctrl+S - Save document
Ctrl+O - Open document
Ctrl+F - Find text
Ctrl+P - Print
Alt + First letter in menu item (sometimes other letter) - Open menu (i.e. Alt+F opens the File menu)
Ctrl+Tab - Switch between open documents
Ctrl+F4 - Close document

Alt+F4 - Close active application
Alt+Tab - Switch between open applications
Alt+Shift (Default, but can be changed in keyboard settings) - Switch between installed keyboard input languages
Alt+Print Screen - Copy only the contens of active window to clipboard

The following apply to various text editors:

Ctrl+H - Replace text
Ctrl+Home / Ctrl+End - Go to the beginning/end of document (can be used in combination with Shift for selecting)
Ctrl+Left Arrow / Ctrl+Right Arrow - Move to next word left/right (can be used in combination with Shift for selecting)

Windows 7 or higher specific

Win+X - Quick menu
Win+Up Arrow - Maximize window
Win+Down Arrow - Un-maximize or minimize window
Win+Tab - List view of all open apps
Alt+Shift+Print Screen - Switch between High Contrast and Normal modes
Win+R - Run a command
Win+G - Game Bar - Start recording a video of your game or some apps that allow recording

Zooming in/out in browsers and text editors

Internet browsers support both:
Ctrl + Mouse wheel up/down
Ctrl + +(NumPad) / Ctrl + -(NumPad),
but some text editors only support the mouse method, others support none.

I know there's more, but I find these to be the most relevant...

If you learned three or more useful shortcuts,
with regards to the time and work they're going to save you,
the least you can do is buy me a drink of some sort :)