Instead of a feature, this time I'd like to introduce you to some shortcuts in Photoshop that really help me save time and a lot of my nerves.
I make use of all of these shortcuts (and a couple more), so even if for some of you these look too complicated, they give you power over Photoshop. By employing these, I don't have to leave the canvas to go search for a function in the depths of Photoshop's many menus.
While I think that these shortcuts can be used universally in all versions of Photoshop, I can only account for CS4 to CS6, maybe CS3.
If you've been frustrated by Photoshop's inability of undoing steps with Ctrl+Z, here's your answer.
F - Toggle Screen Modes
To clear away all the excess panels that you don't need while painting. If you have a second screen, you can put some of your panels there where they will stay visible.
You can change the colour surrounding your canvas by right clicking there. I use black, so that there is less strain for my eyes and I can judge the colours better. If you have trouble with values, you may want to stick to 50% gray.
After pressing F twice, you're in fullscreen mode. Press Enter to get access to your options bar (the one that let's you change opacity and flow among others, if the brush tool is selected.)
Number keys 1-0 with brush tool active
Changes opacity of your brush: 1 equals 10%, 0 equals 100%. Pressing two number keys one after another will select that exact opacity. So, if I press 6 and 7, I get 67%.
Number keys 1-0 with move tool active
Changes opacity of the selected layer (unless you're on the background layer). I actually don't use that one, but knowing this will help you avoid confusion, if all of a sudden your layers go wild.
Brush size and hardness
Space+Ctrl+Right click dragging left and right (since CS5)
Changes the brush size - left makes it smaller, right larger. This method is imprecise, but quick. Those who don't have CS5 or higher can use the bracket keys on QWERTY keyboards. (I think it's those - I use a different layout, so don't pin me down on that.)
On QWERTZ keyboards it's Ä and #.
Space+Ctrl+Right click dragging up and down (since CS5)
Changes the hardness of the brush (if it's not a custom brush, that is).
There are shortcuts for this, too, which you can assign to your liking.
Colours and assigning shortcuts
D - Default Foreground/Background Colour
This will set your colours to black for the foreground and white for the background.
X - Switch Foreground/Background Colours
Shift+Right Click with the brush tool active
Opens up a pulldown with all your brush blending modes available - if you aren't very familiar with these modes yet, keep in mind that there are blending modes for brushes and blending modes for layers. So make sure to not confuse them.
Brush blending modes only affect pixels on the selected layer. So if you try to paint with your brush in multiply mode on a blank layer set to normal the first stroke will only lay down colour.
You'll have to assign that one yourself via Edit/Keyboard shortcuts...
Select Shortcuts For: Tools in the pulldown menu and scroll all the way down. There are two entries: Foreground Colour Picker and Background Colour Picker.
Unfortunately, you cannot assign a combination of keys to the tools; it has to be a single key.
Mirroring the canvas
Same deal as with the colour picker. You can find this command in Shortcuts For: Application Menus/Image/Image Rotation/Flip Canvas Horizontal.
L - Lasso tools
With the lasso selected, make sure that "Anti-alias" is checked in your options bar right below the menu bar. Depending on your preferences and canvas size you can add a feather of 0,5px.
M - Marquee tools
Check Anti-alias if it isn't yet.
Q - Quick Mask Mode
Used to create or modify a selection by painting - you can easily control feather, opacity and structure of your selection. While in this mode, you are painting in grayscale. Painting black will select pixels, while white subtracts pixels from your selection.
Exit the mode by pressing Q again.
Ctrl+H - Hide selection
Hides selection until you deselect with Ctrl+D or press Ctrl+H again. Works during transformation too.
Ctrl+Shift+H - Invert selection
Don't mix this up with Ctrl+I which will invert the colours.
Use this shortcut with an active selection to fine-tune it - while painting, I mostly use it to globally feather the edges, the other options are more useful for photomanipulations.
Now, let's mess with our layers:
Groups all selected layers.
Duplicates the selected layer OR the pixels in a selection on the selected layer. Duplicating multiple layers with this is only possible since CS6, as far as I know.
Only works with an active selection. Cuts out the selection and puts it on a new layer.
Creates a blank layer on top of the selected one.
Merges the selected layer with the one below it OR merges all selected layers.
Creates a merged copy of all visible layers and puts it on top of the selected layer. Doing that beneath one or more adjustment layers isn't a good idea.
Copies every visible pixel in your selection regardless of how many layers you have. Ctrl+V to paste. Again, keep an eye on adjustment layers.
Okay, that's it for now. I sincerely hope this made your head spinning. Depending on your use of shortcuts, these may seem a lot to learn at once, so I suggest making a list of those that you want to learn, and spread the learning process out over a few days, tackling one after another. It's just a suggestion, though, so do whatever works for you. If it's a bigger topic, I'll also consider writing a journal or doing a tutorial.
Also, feel free to share this information - I'd be grateful if you linked back here, of course, especially if you copy whole passages.
If you encounter any problems or questions about this or anything other related to Photoshop or digital painting, just comment or drop me a note. I'll do my best to provide you with answers or refer you to information on the web or on dead trees.
If you want to add to these shortcuts, please comment, I'm always eager to learn more of these.