⌥⌘O
Warning: extreme Mac Siracusian nerdery ahead.
If you’re an old-school Mac user, then you may prefer your Finder to be set to “Always open folders in a new window.” (Consult your Finder preferences to change this.) Personally, I prefer it that way because I like to arrange certain icons and size certain windows just so, whereas with the default setting, folders open in the same window and icons arranged according to whatever the settings happen to be at the time. (This is also one thing that infuriates me about using Windows.) My Home folder is sized the way I want, with the icons the size I want and in the order I want. That sizing makes no sense for any of the subfolders, however.
Nevertheless, this setting leads to one problem — an almost comical proliferation of windows on your desktop every time you go digging for files the old-fashioned way. (The modern way would of course be to simply use Spotlight, but sometimes one doesn’t know what the file is called that one is looking for, and not all files can be indexed by content.) Since I also prefer a clean desktop, I generally like to close each folder window as I open one of the folders inside.
Before Leopard — going all the way back to before there was even OS X — you could get a window to close itself while you opened a folder by holding down the Option key as you double clicked — or if you’re a keyboard-happy person, as you pressed Command-O or Command-Down. Same went for Command-Up (to open the parent folder). The old window would disappear and the new window would open at the size and location you last left it.
In OS X 10.5 (Leopard), Apple mucked around with the Finder, and made it so that the Option key no longer did this useful thing. Instead, holding down Option would open the new folder in the same window, and thereby merely replace the contents but keep the same size, location and icon settings. Basically they made the Option key a way to temporarily reverse the entire preference for new windows. I did not like this, and I reported it as a bug early in Leopard’s existence. Apple even acknowledged it as a bug (rather than an intentional change), but it didn’t get fixed through Leopard’s entire eleven-version release cycle.
Happily, Snow Leopard returns some, though not all, sanity to the Option key shortcut. Command-Option-O will open a folder in a new window and close the current window.
What Apple still hasn’t fixed (and I’m guessing they never will now) is ⌥⌘↓ and ⌥⌘↑. Holding down Option with these shortcuts still merely causes them to replace the current window rather than the close/open dance.
