Where is the data folder for Bitcoin-Qt? Where can I find the blockchain, wallet.dat, etc. on each of the operating systems Bitcoin-Qt supports?
It is the same data folder as the non-QT version of the standard client used to use:

Linux:

~/.bitcoin/

MacOS:

~/Library/Application Support/Bitcoin/

Windows:

%APPDATA%\Bitcoin

(This defaults to C:\Documents and Settings\YourUserName\Application data\Bitcoin on Windows XP and to C:\Users\YourUserName\Appdata\Roaming\Bitcoin on Windows Vista, 7, 8, and 10.)