I'm sure you know some of these already, but these are some of the most used ones.
Command + Q : quit app
Command + S, Command + shift + S : save and save as
Command + C, Command + V, Command + X : copy, paste and cut selection
Command + tab : switch apps (hold to see all and tap tab to cycle or tap both to switch to last app)
Command + ~ : switch windows within a app
Command + T : new tab (mostly for web browsers)
Command + N : new window
Command + W : close window (or tab if in an app with open tabs like a web browser)
Command + shift + 3 : take a full screenshot (saved to the desktop)
Command + shift + 4 : take a custom sized screenshot, you can also tap the space bar while in the mode to snap the size to a window
Command + shift + N : new folder
space bar (in Finder) : while having a file(s) it toggles QuickLook on it, try it on a bunch of different file types (movies, pictures, documents etc.) and you'll see how cool it is
Command + space bar : open Spotlight (the search tool in the upper right hand corner of the screen)
Command + delete (in Finder) : send to trash
Command + shift + delete (in Finder) : empty trash
This is just the tip of the iceberg, and you can always set your own. If I come up with any more really good ones I'll post them.