OS X comes with some really nice keyboard shortcuts to take screenshots. The problem is that I keep forgetting exactly which shortcut does. So for the purpose of having a place to look them up the next time I forget - here they are

Saving to a file on the desktop

• ⌘-Shift--3: Take a screenshot of the screen
• ⌘-Shift--4, then select an area: Take a screenshot of an area
• ⌘-Shift--4, then Space, then click a window: Take a screenshot of a window

Saving to the clipboard

• ⌘-Ctrl-Shift--3: Take a screenshot of the screen
• ⌘-Ctrl-Shift--4, then select an area: Take a screenshot of an area
• ⌘-Ctrl-Shift--4, then Space, then click a window: Take a screenshot of a window