If the videos are available for capture, you can sometimes Control-Click inside the movie window and the dialogs will ask you for a capture destination, or, if the system opened up QuickTime Pro (and you have QuickTime Pro), you might be able to export the video after it finishes downloading and before it finishes playing.
If none of that works, then Google is using a propriatary internal movie player and you're stuck. Sometimes the PC people have better tools for that kind of thing than the Mac people.
Koz