Change Brush Tip Sizes Quickly
With a brush selected, simply Press the Right Bracket Key "]" to increase the size of your brush tip, or Press the Left Bracket Key "[" to decrease the size of your brush tip.

The Move Tool & The Arrow Keys

Get in the habit of calling up the Move Tool with the keyboard shortcut, which is the letter V. Also, once the Move Tool is active you can use your Keyboard Arrow Keys to nudge a layer or selection in 1 pixel increments. To speed things up, Press Shift along with an arrow key to nudge in 10 pixel increments.

Hide Your Palettes
Press Tab once to hide all your palettes. Press Tab again to bring them back. Press Shift+Tab to hide all your palettes except the toolbar.

Zooming In & Zooming Out

To zoom in Press Command and the plus sign "+" (PC: Control and the plus sign). To zoom out Press Command and the minus sign "-" (PC: Control and the minus sign).

Navigating In Magnified Images
If you've zoomed in on a large image and have lost your bearings you can jump quickly to specific views using these shortcuts:
Press the Home Key to set the view to the top left hand corner of your image.
Press the End Key to set the view to the bottom right hand corner of your image.
Press the Page Down Key to move the view one full screen down.
Press Command+Page Down (PC: Control+Page Down) to move the view one full screen to the right.
Press Command+Page Up (PC: Control+Page Up) to move the view one full screen to the left.

Selecting Just The Pixels On A Layer

An easy way to select an object that is on a transparent layer is to Press the Command Key (PC: Control key) and click on the layer with the object in the Layers Palette. This makes sure that only the opaque pixels (the pixels that are visible) will be selected with the marching ants, instead of the entire layer.

Tile Images For Better Visibility
When you open many files at once Photoshop has them cascade, and the overlapping files can make it difficult to select individual ones. To get around this choose Window> Arrange> Tile and all of your open files will rearrange themselves to be visible all at once. When you're ready to close them all don't spend time closing them individually, instead use the Close All shortcut Command+Alt+W (PC: Control+Option+W).

Getting Rid Of The Welcome Screen - And Bringing It Back

When first launching Photoshop CS you'll see a Welcome Screen that has links to some Tutorials, some Tips and Tricks, and a set of Color Management Help Files. After a while you might want to get rid of this screen. There's a check box at the bottom you can select to hide the screen at startup.
If you change your mind later and decide you'd like to investigate the tutorials and other goodies, you can temporarily bring the screen back by going under the Help menu and choosing Welcome Screen.

Cycle Through All Of Your Open Documents
Sometimes you'll want to find one of your open documents that is hidden, but navigating using the Window menu won't help because you can't recall the name of the file. Instead press Control-Tab repeatedly (MAC and PC) to cycle through all of your open documents.