If you use Safari Internet browser on your OSX box, you can easily and freely download flash videos such as Youtube. Not only that, but you can also use Safari as an HTTP analyzer to help you debug HTTP problems.
Once you are inside the page serving the flash video, press 'Command-Option-A' to enter the Activity Resource. In this window you can see all the downloaded files when you accessed a given page, including downloaded flash videos stored in your browser's cache:
In the above session we have located the file matching the flash video by looking at its size (6.6MB) - in addition to that you can notice that this video stream was still downloading at the time of the screenshot! The file can be downloaded by double clicking the entry.
You can also notice all the other files which constitute the youtube page we were accessing. This is very useful to debug, hack or analyze HTTP traffic.