Windows: Windows only lets you take simple screenshots out of the box, and its built-in screencasting tool doesn't even output a standard video file. oCam is a free utility that solves both of these problems.
oCam overlays a green box on your screen that you can resize and move as you see fit. The capture button below the box will take a screenshot of the selected area and place it in an oCam folder inside of My Documents. If you need to do some tech support for relatives, the record button will start recording a screencast within the green box, and you can choose from a variety of video codecs. It would be great to see oCam add some cloud uploading options, but it's still a huge step up from Windows' built-in tools.
oCam is free, so it's really a no-brainer. If you need something similar for Mac OS X, be sure to check out previously-mentioned Screeny.