Mac
 
    ALT + CMD + SPACE to search in finder
    CMD + ALT + Left/Right to switch between tabs
    CMD + L to access the input field
    CMD + ` on a mac to tab through application windows
    CMD + OPT + ESC to Force Quit apps
    CMD + Tab to cycle through open apps on Mac
    CMD + H to hide the window
    CMD + ALT + H to hide other windows
    Shift + CMD + I when in safari opens a mail with the link of the current page in it
    CMD + T followed by CMD + K to perform a search in a new tab
    ALT + CMD + Eject put the mac to sleep
    CMD + Shift + A to autofill forms in Safari
    OPT/CMD + arrow keys – also hold Shift to select. Makes it easy to delete multiple words/lines quickly
    CMD + OPT + Shift and V to paste without coping the formatting of what you pasted
    CTRL + OPT + CMD + 8. Inverts Mac screen colors
    CMD + Shift + 4. Saves a snapshot of a selected area to the desktop (You can then press spacebar to cycle between a crosshair or window selection)
    CMD + CTRL + Shift + 4 = snap a screen a picture of a marquee area of the screen and copy it to clipboard
    CMD + SPACE to open spotlight
    CTRL + OPT + CMD + Eject Quits all applications and shuts the computer down
    CMD + E to eject selected volume on the Mac
    CTRL + Shift + CMD + 3 for clipboard – screenshot, compared to print-screen on PCs
    CMD + , on a Mac to access current app preferences
    CMD + OPT click and drag creates an alias of the file where you drag it
    CMD + Delete to move to trash
    CMD + Shift + Delete to empty the trash from Finder
    CTRL + OPT + CMD + . = increase contrast CTRL + OPT + CMD + , = decrease contrast
    CMD + OPT + D to hide/unhide the dock
 
Windows
 
    CTRL + INS to copy, Shift + INS to paste, Shift + Delete to cut. Such an underappreciated series of hotkeys.
    CTRL + 0 to return web page to 100% (default) zoom level
    CTRL + Shift + Eject to turn off monitor
    CTRL + Shift + ESC on windows to get to task manager directly
    Win7: WIN + M to minimize all opened windows
    WIN + Left/Right to put windows side by side. Nice live coding method.
    CTRL + K + D this hotkey indents html and c# code nicely
    WIN + D (return to desktop)
    ALT + BACK to backspace by the word rather than by the character
    win: Shift + Delete delete files without saving to trash bin
 
Linux
 
    CTRL + ALT + T to open Terminal on Ubuntu. I can’t live without it now
 
Photoshop / Illustrator
 
    CMD + Shift + C in Photoshop to copy the merged selection (save’s you the step of merging your file then undoing!)
    In Photoshop – CMD + ALT + 0 to resize the window to 100% is essential for web designers!
    Shift + ALT + CMD + S (win: CTRL + Shift + ALT + S)… Save for web in Photoshop
    CTRL + Shift + C flatten and copy with transparency, Photoshop
    Illustrator CTRL + 7 to crop image
    Photoshop: CTRL + Shift + ALT + E merges all visible layers to a new layer on top of the others without deleting them
 
Firefox / Chrome
 
    CTRL + Shift + E for Edit CSS (Firefox/Webmasters Toolbar)
    CMD + Shift + C Inspect Element // Firefox—Firebug // Chrome—Developer Tools
    CTRL + Shift + T to open closed tabs FF Chrome, and CTRL + Shift + N to open closed windows FF
    Firefox CTRL + L to enable retyping of a web address
    ESC hides the mousepointer (in browser). Great for Screenshots
 
Honorable Mention: Mouse Gestures
 
    “I almost stopped using hot keys when started using mouse gestures, both in ff and chrome”
 
    Press Shift and scroll your mousewheel. Scrolls horizontally on the page
    Click on links using mousewheel in FF & Chrome and links open automatically in new tab