Mac OS X keyboard shortcuts:

Please Note:

Hold down the modifier keys, and then press the letters...

These are default keystrokes -unless specified. You can change them using the 'Keyboard & Mouse' Preference pane....

"Object" infers a highlighted, or selected item (folder, or icon).

Command-A [Edit menu] Select All.

Shift-Command-A [Finder; Go menu] Open Applications folder.

Command-B [View menu] Hide toolbar.

Command-C [Edit menu] Copy object.

Option-drag Copy dragged files to new location.

Command-drag Move dragged files to new location.

Option-Command-drag Create alias (shortcut) of dragged files/folders.

Shift-Command-C [Finder; Go menu] Open Computer window.

Command-D [File menu] Duplicate object.

Control-Command-D [Dictionary] Define selected or hovered word.

Option-Command-D Toggle hide/show Dock.

Command-E [File menu] Eject media; CD, DVD, Mounted volume...

Command-F [File menu] Find...

Shift-Command-F [Finder; Go menu] Open favorites folder.

Shift-Command-G [Finder; Go menu] Goto... folder.

Command-H [App menu] Hide (focused, or top) application.

Option-Command-H Hide other (not-focused) applications.

Shift-Command-H [Finder; Go menu] Open Home folder.

Command-I [File menu] Get info on object.

Option-Command-I Show attributes Inspector.

Shift-Command-I [Finder; Go menu] iDisk.

Shift-Command-I [Safari] Send current page to mail (New message.)

Command-J [View menu] Show view options on object.

Command-K [Go menu] Connect to server...

Command-L [File menu] Create alias (shortcut, or pointer to object).

Command-M [Window menu] Minimize (focused, or top) window.

Option-Command-M Minimize all (focused or top applications) windows to dock.

Command-N [File menu] New Finder window.

Shift-Command-N [File menu] New folder.

Command-O [File menu] Open...

Command-P [File menu] Print.

Command-Q [App menu] Quit (focused, or top) application.

Shift-Command-Q Quit all applications and log-out.

Shift-Option-Command-Q Log-out without warning.

Command-R [File menu] Show original object.

Command-S [File menu] Save project.

Shift-Command-S Save as...

Command-T [File menu] Add object to favorites.

Command-V [Edit menu] Paste (previously copied) object.

Command-W [File menu] Close (focused, or top) application window.

Option-Command-W Close all windows.

Shift-Option-Command-W Close all windows and dialog boxes.

Command-X [File menu] Cut object.

Shift-Command-Y Creates a new sticky note from selected text.

Command-Z [File menu] Undo.

Command-Delete [File menu] Move to trash.

Shift-Command-Delete [Finder menu] Empty trash.

Shift-Option-Command-Delete Empty trash without dialog box.

Command-1 View as icons.

Command-2 View as list.

Command-3 View as columns.

Shift-Command-3 Screen snapshot to desktop.

Shift-Control-Command-3 Copy entire screen to clipboard.

(Escape to cancel. Pictures saved as TIFF files; PDF files for Mac OSX 10.2 & newer.)

Command-4 View in Cover-flow.

Shift-Command-4 Snapshot of selected area to desktop.

Shift-Command-4-Space Snapshot of window to desktop.

Shift-Control-Command-4 Copy selected area to clipboard.

(Escape to cancel. Pictures saved as TIFF files; PDF files for Mac OSX 10.2 & newer.)

Option-Command-Esc Force Quit dialog box.

Shift-Option-Command-Escape (hold 4 seconds) Force quit current application.

Command-. (period) Stop process.

Control-Eject Dialog box; Restart, Sleep, Shutdown.

Shift-Control-Eject Put display to sleep.

Control-Command-Eject Quit all Apps and Restart.

Option-Command-Eject Sleep Now.

Option-Command-Power Sleep Now.

Control-Option-Command-Eject Shut down immediately.

Command-? [Help menu] Open Mac help.

Command-[ [Finder; Go menu] Back.

Command-] [Finder; Go menu] Forward.

Option-left arrow [Editing text] Move to beginning of current or previous word. (Hold Shift to select through.)

Command-left arrow [Editing text] Move to beginning of line. (Hold Shift to select through.)

Command-left arrow Collapse folder in list view.

Option-right arrow [Editing text] Move to the end of current or next word. (Hold Shift to select through.)

Command-right arrow [Editing text] Move to end of line. (Hold Shift to select through.)

Command-right arrow Expand folder in list view.

Option-Command-right arrow Expand folder including nested sub-folders in list view.

Command-down arrow [Editing text] Move to end of all text. (Hold Shift to select through.)

Command-up arrow [Editing text] Move to beginning of all text. (Hold Shift to select through.)

Option-Command up arrow Open parent folder while closing existing window.

Option-Space Inserts a "non-breaking space" into a document.
Command-Space Open Spotlight.

Command-Alt-Space Open Spotlight Guide.

Function-Delete "Forward Delete"

(Video/Audio & Miscellaneous Shortcuts:)

Option-Function-1 Open Displays Preference pane.

Command-Function-1 Switch between video mirroring and extended desktop screens.

Option-Function-2 Open Displays Preference pane.

Command-Function-2 Auto-detect new display.

Option-Function-3, 4, 5 Open Sound Preference pane.

Function-8 Toggle Spaces display.

Shift-Function-8 Toggle Spaces display -slow motion.

Function-9 Show all open windows.

Shift-Function-9 Show all open windows -slow motion.

Function-10 Show all windows for focused or top application.

Shift-Function-10 Show all windows for focused or top application -slow motion.

Option-Function-8, 9, 10 Open Keyboard and Mouse Preference pane.

Function-11 Minimize all windows.

Shift-Function-11 Minimize all windows -slow motion.

F12 Eject media.

Function-12 Open / Close Dashboard.

Shift-Function-12 Open / Close Dashboard -slow motion.

F14 Dim display.

F15 Brighten display.

Option-Command-S (While viewing video) Creates a picture from the playing video, or iChat.

Command-click and drag to send a picture clipping to the finder, or another application.

Option-quick look (Customized window) Display full screen.

Hold down Control while using a scroll-mouse to zoom the screen in and out.

From the comments:

Concerning dialog [pop-up operating system] boxes:

Pressing the 'Esc' - escape key will select the 'Cancel' button.

Pressing 'Tab' to move between options.

Pressing 'Return' to select the highlighted option.

Thanks Nicolas!