I just noticed this thread now.
In OS X, press Cmd-Shift-G to access the "Go To..." dialogue; very useful to access hidden folders that you know the directory of.
Accidentally dragged a volume off of the Finder sidebar and have no way of getting it back? No worries! Press Cmd-Shift-G and type in "/Volumes". This also can show you all of the volumes/disk images currently mounted on your computer.
To switch active windows in OS X, press Command-`. This cycles through all open windows. To cycle backwards, press Command-Shift-`.
To hide an application so the world can't see it, press Command-H. You can also do this via the application menu. NOTE: The indicator still shines, so be careful around tech-savvy people who know OS X.
To start up in verbose mode in OS X (it looks cool), press Command-V before the grey Apple logo appears (only on a real Mac; Hackintoshes that use Chameleon 1 or any other non-graphic bootloader can access this by typing -v into the boot prompt and Chameleon 2 bootloaded computers have this as a drop-down menu option). It'll tell you everything that's happening during boot, whether you understand it or not. It also displays verbose logs on logout and shutdown.
To choose a bootable device at boot, hold Option before the boot prompt appears. To select something as the default boot drive, hold down Command when selecting the drive.
To get the properties of any file, folder, or volume, select it and press Command-I.
To navigate the Finder via the keyboard, press Command-N while Finder is selected to open a new window. To select folders or files, just use the arrow keys; to open it, press Command-O or Command-Down. To go up a level in the folder hierarchy, press Command-Up. To go forwards/backwards in history, press Command-] or Command-[ respectively. To close a window, press Command-W. I'm not entirely sure how to get to the sidebar, though.
...I have more, just need to think of them.