Here is the list of the most important shortcuts to know:

Option: Display all bootable volumes (Startup Manager)
Shift: Perform Safe Boot (start up in Safe Mode)
C: Start from a bootable disc
T: Start in FireWire target disk mode
N: Start from NetBoot server
X: Force Mac OS X startup (if non-Mac OS X startup volumes are present)
Command-V: Start in Verbose Mode
Command-S: Start in Single User Mode
Command-A: Select all items in the front Finder window (or desktop if no window is open)

Option-Command-A: Deselect all items
Shift-Command-A : Open the Applications folder
Command-C: Copy selected item/text
Shift-Command-C: Open the Computer window
Command-D: Duplicate selected item
Shift-Command-D: Open desktop folder
Command-E: Eject
Command-F: Find any matching Spotlight attribute
Shift-Command-F: Find Spotlight file name matches
Option-Command-F: Navigate to the search field in an already-open Spotlight window
Shift-Command-G: Go to Folder
Shift-Command-H: Open the Home folder of the currently logged-in user account

Command-I: Get Info
Option-Command-I: Show Inspector
Control-Command-I: Get Summary Info
Shift-Command-I: Open iDisk
Command-J: Show View Options
Command-K: Connect to Server
Shift-Command-K: Open Network window
Command-L: Make alias of the selected item
Command-M: Minimize window
Option-Command-M: Minimize all windows
Command-N: New Finder window
Shift-Command-N: New folder
Option-Command-N :New Smart Folder
Command-O: Open selected item
Shift-Command-Q: Log Out

Option-Shift-Command-Q: Log Out immediately
Command-R :Show original (of alias)
Command-T: Add to Sidebar
Shift-Command-T: Add to Favourites
Option-Command-T: Hide Toolbar / Show Toolbar in Finder windows
Shift-Command-U: Open Utilities folder
Command-V :Paste
Command-W :Close window
Option-Command-W: Close all windows
Command-X: Cut

Option-Command-Y: Slideshow (Mac OS X 10.5 or later)
Command-Z: Undo / Redo
Command-1: View as Icon
Command-2: View as List
Command-3: View as Columns
Command-4: View as Cover Flow (Mac OS X 10.5 or later)
Command-,: (Command and the comma key) Open Finder preferences

Command-`: (the Grave accent key--above Tab key on a US English keyboard layout) Cycle through open Finder windows
Command-Shift-?: Access Mac Help
Option-Shift-Command-esc: Force Quit Finder
Command-[: Back
Command-]: Forward
Command-Up Arrow: Open enclosed folder
Control-Command-Up Arrow: Open enclosed folder in a new window

Command-Down Arrow: Open highlighted item
Command-Tab: Switch application -cycle forward
Shift-Command-Tab: Switch application -cycle backward
Command-Delete: Move to Trash
Shift-Command-Delete:Empty Trash
Option-Shift-Command-Delete: Empty Trash without confirmation dialog

Spacebar (or Command-Y): Quick Look (Mac OS X 10.5 or later)
Command key while dragging: Move dragged item to other volume/location (pointer icon changes while key is held - see)
Option key while dragging: Copy dragged item (pointer icon changes while key is held)
Option-Command key combination while dragging: Make alias of dragged item (pointer icon changes while key is held)