Just a handfull of the keys, that exist, and you can always adjust short cuts to your own liking in the programs too.
But these are some of the ones I use the most myself. Found their equals for Mac too, so thought I might as well post that.
Shortcuts - Photoshop
Merge visible Layers
MAC: Cmd+Shift+E
WINDOWS: Ctrl+Shift+E
Invert selection
MAC: Cmd+Shift+I
WINDOWS: Ctrl+Shift+I
Select all layers
MAC: Cmd+Opt+A
WINDOWS: Ctrl+Alt+A
Deselect the entire image
MAC: Cmd+D
WINDOWS: Ctrl+D
Select a colour from an image (color picking)
MAC: Opt+Brush Tool
WINDOWS: Alt+Brush Tool
Increase/decrease brush s