InstantStorm is a screensaver creator that uses SWF files. It allows you to put flash videos on your screensaver. The only format that it does support so far is SWF. Although there aren't many videos still released or found in that format, there are many converters that can convert pretty much anything to .SWF files.
The application is completely free, even for commercial use. It allows you to design and customize pretty much every aspect of the screensaver. You can add a main SWF that will be displayed with the screensaver and then add various BMP or SWF files for the settings dialogs.
There are two ways in which you can create a screensaver from scratch. You can do it manually, which allows you to change most of the settings, or use a wizard. The latter is relatively easier than using the manual mode, but all you really do is add .SWF file and set up the installer. In the manual mode, you can even customize the installation wizard for your screensaver, which will be saved to an .EXE file that will place the screensaver file in the appropriate path.
I have seen some very good screensavers created with InstantStorm. You can even create HD video screensavers with this tool, but the size of the screensaver will be huge.