Source: http://lifehacker.com/5620616/imagegrab-is-a-powerful-video-screenshot-tool
Windows: Grabbing stills from a video using a traditional screenshot tool is a tedious hit-or-miss affair. ImageGrab is feature-packed tool for grabbing single stills, a series at set intervals, or scripting your own parameters for custom grabs.
ImageGrab makes it easy to precisely snap a frame directly from the source video itself. You can automate the process by setting ImageGrab to grab frames at a user-specified interval and stamp the grabs with information like the name of the file and where in the original video the grab is from.
If the default settings and toggles aren't versatile enough for you, ImageGrab supports user scripting. We tested both the portable and stable release ImageGrab 4.2 as well as the non-portable and beta of ImageGrab 5.0. Although we were disappointed the beta wasn't portable it was significantly more stable on our Windows 7 test machine than the older 4.2 version.
ImageGrab is freeware, Windows only. Have a favorite tool for snagging stills from a video? Let's hear about it in the comments.