Screenshot

Screenshots are images taken in-game by pressing the F2 key (Fn+F2 for Macs). They can be found in the screenshots folder of the .minecraft directory, which is found by pressing "options" in the Minecraft launcher. Screenshots save as a PNG file with the following format: YYYY-MM-DD_HH.MM.SS.png, for example: 2011-02-12_12.25.05.png, which translates to February 12, 2011, 12:25:05. The screenshot will save exactly what it looks  like in-game, so if you press F1 then F2, all of the GUI will be hidden in the screenshot.

The screenshots folder location varies by operating system.
 * Linux: ~/.minecraft/screenshots
 * Windows XP: %appdata%/.minecraft/screenshots
 * Windows Vista and up: %appdata%/Roaming/.minecraft/screenshots
 * Mac OS X: ~/Library/Application Support/minecraft/screenshots

Trivia

 * If you try to screenshot a render hole, the game will crash.
 * Before the Beta 1.5 update was implemented, holding shift while pressing F2 will create a massive screenshot which would crash the game.