1. Drag-ZoomCommand/Ctrl + Space + Click and Drag Left or Right. Fastest way to zoom in and out with one keyboard shortcut (only in CS5 and only with GPU enabled).
2. HUD Color PickerAlt + Shift + Right Click (Win) or Control + Option + Cmd + Click (Mac). Gorgeous color picker with a color wheel (only in CS5 and only with GPU enabled). Preferences are under General tab.
3. Bird’s Eye ViewHold Down H + Click, Reposition View and Let Go of the Mouse to Zoom Back). Works great with large images, when you need to work closely. Fast way to fit the image to screen and then zoom back into another area with the previous zoom ratio quickly.
4. Brush Size & HardnessCtrl + Alt + Right Click (Win) or Control + Option + Click (Mac). Drag your mouse to right/left to change the size and up/down to change the hardness. It only works with CS5 in both directions. In CS4 only the size change works. You can change the color of the brush preview under Preferences/Cursors. You also need GPU turned on under the Performance tab to be able to see the brush preview.
5. Draw Straight LinesHold Down Shift. Works with nearly every drawing tool.
6. MoveHold Down Command/Ctrl. Fastest way to move something quickly and then switch back to the previous tool.
7. Pan (Shift + Space) and Zoom Simultaneously in all DocumentsShift + Ctrl + Space (Win) or Shift + Command + Space (Mac). Best way to compare or work on more than images at the same time.
8. Hide PanelsHit Tab. You can always show them again by pressing Tab once more, or just simply hover over the edges to show the panels and toolbar temporarily.
9. Full Screen Preview of Files From Mini-BridgeSpace and Then Arrow Keys. It works the same way as in Bridge. Make sure that you deselect the images in MiniBridge or collapse the panel when you continue working in Photoshop.
10. Text SizeCommand/Ctrl + Shift + . or , you can use it for selected text. It won’t work by only selecting the text layer.
11. All Caps for TextCommand/Ctrl + Shift + K. Quick way to switch between lowercase and uppercase text. Make sure you type without Caps lock to be able to use this.
12. Tracking and KerningAlt/Option + Left and Right Arrow Keys. Useful way to make equalize the distance between characters. If you select a word, a line or a whole paragraph then you change the Tracking. If you click between two characters you change the Kerning.
13. LeadingAlt/Option + Up and Down Arrow Keys. Quickly change the distance between selected lines.
14. Baseline ShiftAlt/Option + Shift + Up and Down Arrow Keys. Easy way to use one text layer but have different positions for the text.
15. FillCommand/Ctrl + Backspace – Foreground Color, Alt/Option + Backspace – Background Color, Shift + Backspace – Fill Options. Great way to fill color into selections or change the color of text and vector shape layers.
16. Reset Any Dialog BoxAlt/Option + Click on Cancel. Instead of canceling a dialog box and returning to it try Reset and save time and effort.
17. Changing Opacity and Fill OpacityNumeric Keys, Shift + Numeric Keys. This will change the opacity and fill of the selected layer (or even more than one layer at the same time in CS5) and it will change opacity and fill of the tool if you select a drawing tool.
18. Invert Colors of Selected Layer or Layer’s Pixel MaskCommand/Ctrl + I. This is a fast way to invert visible parts of a layer if you invert the colours of the layer mask.
19. Switch to Precise Cursors for Drawing ToolsHit Caps Lock. If you don’t want to see the edges of the brush you can switch to precise mode easily by having Caps lock on.
20. Reset PreferencesCtrl + Alt + Shift (Win) or Command + Option + Shift (Mac) When Loading Photoshop. If you want to start with the default settings and user interface hold down these buttons while Photoshop is loading.
21. Switch Foreground/Background ColoursPress (X). Fast way to switch between the selected colour.
22. Mask ViewAlt/Option + Click on a Layer Mask. Great way to clean up the mask after you save it from a selection. Using it again will switch back to normal view again.
23. Disable MaskShift + Click on a Layer Mask. You can quickly show the original contents of the layer with this keyboard shortcut. Using it again will enable the mask again.
24. Intersection of SelectionsAlt/Option + Shift + Click With a Selection Tool Over Another Overlapping Selection. Easy way to keep only a specific part of a selection.
25. Merge All Visible Layers to a New LayerCtrl + Alt + Shift + E (Win) or Command + Option + Shift + E. If you want to keep all your layers but you also need to merge them together to be able to use something you can convert them all into one Smart object or use this keyboard shortcut to make a duplicate merged layer.
Bonus (Easter-Egg) Keyboard ShortcutAlt/Option + Click on Panel options under the options of Layers panel.
ConclusionOf course you can use Photoshop without keyboard shortcuts, but you can be much faster and productive if you spend a little time on getting used to have one of your hands over the keyboard all the time and memorize some of these combinations. Feel free to include some other useful shortcuts in the comments section.