GizmoFlicks works well with YouTube, Google Videos, and Metacafe. Sometimes it works on Yahoo videos. We are still working on support for Yahoo so check back with us from time to time to see if we have it working.
We recommend using a third party video player such as VideoLan. Its a free open source program that supports Linux, Mac and Windows (just to name a few). You can find it here: http://www.videolan.org/.
There are several good video converters out there. For windows, we recommend that you use WinFF which can be found at http://winff.org/html_new/.
For Linux and Mac OS X, we recommend using FFMpeg which can be found at http://www.ffmpeg.org/