I have a video, and I want to pull out a certain frame from it, as an image.
Is there any free software that can step frame-by-frame through a video, and then extract a specific frame to an image?
Since you did not specify an operating system, I'm assuming you're talking about Microsoft Windows.
The first thing that comes to mind is Virtualdub, as I have used it myself for exactly the same purpose.
Virtualdub may lack the editing power of a general-purpose editor such as Adobe Premiere or Sony Vegas, but it is streamlined for fast linear operations over video and it has batch-processing capabilities for processing large numbers of files (including exporting single frames, which you asked for). Last but not least, Virtualdub can be extended with third-party video filters too.
And yes, it's free… licensed under the GNU General Public License (GPL).