As I mentioned a couple of years ago, when you watch a video on websites like YouTube and Dailymotion, the file is actually downloaded to your computer, instead of being streamed. This allows the viewer to replay the video immediately after the initial viewing, even if the first viewing was sporadic because of slow download speeds. This allows you to salvage the video itself from your browser cache. I figured this out when YouTube was in its infancy back in 2006, enabling me to assemble a music video compilation of over 3,000 items.
Now you may wonder why I would want to keep the flash video file itself when you can always go to YouTube and replay the item on demand. There are at least three reasons. First of all, it lets you play the file even if you're offline. Secondly, when you have the file you can navigate forward or backwards through the video as you wish. But most importantly, not everything is forever on YouTube. Indeed, probably most of the music videos that were on YouTube back five or six years ago have been removed as account holders remove files or close their accounts, or more likely, as copyright holders force the removal of the videos.
Unfortunately, actually fishing the file out of the cache has become more difficult of late. Updated versions of Firefox no longer have a central cache file that you can locate and isolate downloaded files. A third party created a Firefox add-on called Cache Viewer that solved the problem, but a subsequent version of Firefox rendered Cache Viewer useless. Had I realized this was the case, I would have not upgraded to the new version of Firefox, but I didn't find out about this until it was too late. At that point I was forced into relying on the Internet Explorer cache, which can be accessed through an arcane series of internet option choices that make your temporary internet files visible, but which is a little less reliable. Then in the only time that a full computer crash turned into a blessing, I was forced to reinstall Firefox and discovered that I had kept a version of Firefox that could still use the Cache Viewer add-on, so I reverted to that version and have been happily harvesting videos in the cache.
However, due to recent changes at YouTube, I think my ability to fish their videos out of my cache may have been thwarted. Recently, I noticed that videos in the browser cache have been divided into multiple files of 1.75 megabytes each, none of which is usuable. Firefox users blame the updated version of that browser for this, but the same thing occurs with Internet Explorer, too, and furthermore this does not happen with other video sites, so I'm guessing that YouTube is behind this. I don't know whether they did this to thwart people like me trying to save a copy of Katy Perry's latest video, or whether it facilitates the upload and play of much larger video files than the old days to accommodate higher quality videos. However it looks like I am now stuck with other video sites, like Dailymotion and Metacafe, which don't divide the files, but which don't have the same selection either.
Note: If you use Firefox, the video-downloadhelper add-on should work.