Wikipedia:Tip of the day/June 20
In Microsoft Windows, a screenshot of the entire monitor, complete with taskbar, can be copied to the system clipboard by pressing the Prt Sc key. Alternatively, pressing Alt+Prt Sc will copy just the active window to the clipboard. You can then paste the clipboard into a program like MS Paint or Paint.NET using Ctrl+V, and then save it as an image file (for uploading to Wikipedia, for instance).
In Mac OS X, simply prepare your screen to how you want it to look, then hit ⌘ Cmd+⇧ Shift+3 (not the number pad 3) simultaneously. The new image will appear as a PNG on your desktop. You can then crop it in Preview (select the area and then hit ⌘ Cmd+K).