I use a freeware tool called Media Player Classic. It looks alot like Windows Media Player.
http://en.wikipedia.org/wiki/Media_Player_Classic
Play any video file. When you find a scene you like just hit "Pause" and under FILE select "Save Image".
Usually I'll save 5-6 images from a video clip and pick the best one. Then I'll crop and sharpen it with Photoshop.