You can also leave an impression on a business meetings and make people think that you are a geniuos, and they will have more trust in you when they see how you are using your computer (i’m not saying taht you are not a genius).
Here are the most common i use in my everyday work:

Cmd + Backspace = Delete selected file/folder
Cmd + Shift + Backspace = Empty the trash
(Finder should have focus for this shortcut to work)
Cmd + Shift+ 3 = Print screen
Cmd + Shift + 4 = Print screen selected area

Fn + Backspace = Delete
Cmd + Arrow (left/right) = move to start/end of line
Cmd + Shift + Arrow (left/right) = select all to start/end of line
Alt + Arrow (left/right) = Move cursor left/right by one word
Alt + Shift + Arrow (left/right) = Move cursor left/right by one word
Cmd + Arrow down = Default action for selected file
(eg. Folder = Open folder, Image = Open image with defult viewer…)
Cmd + Arrow up = Up one folder
Space = Scroll page down
Shift + Space = Scroll page up

Firefox:
Alt+L = Address bar focus
Alt+K = Search bar focus
Shift + Cmd + Backspace = Clear private data
(Firefox should have focus for this shortcut to work)
Cmd + 1 = first tab, Cmd + 2 = second tab …

Las’t but not the least, if you haven’t allready try Quicksilver.
Cheers to developers!