[any ANONYMOUS forum user]

You can do this right from Safari.

1. Go to a flash video page and hold down Option-Apple-A
2. An activity window opens
3. Scan all the processes and look for a the only large file in the list (500 Kb to as much as 10Mb) file that is often still in the process of downloading (sometimes but not always with a .flv suffix)
4. Double click it and OSX downloads it.

If you install the free Qucktime plugin Perian, you can watch the flash files right in quicktime.