You've seen streaming videos on YouTube and elsewhere, but every time you view them they use up some bandwidth and slow the internet down. There is no need for this, however, as these files do actually exist on your computer while you have YouTube displayed in your browser, and you are able to retrieve this file from your cache to keep forever.
How do I do that then?
Every page you visit on the internet goes into your browser cache, a limited amount of space on your computer's hard drive where everything you see gets stored temporarily; this is to save your browser having to fetch them every time you browse the same site, making your browsing experience faster even when you have broadband. All the images, pages, and video clips too, they're all there, but it's not as simple as all that because most browsers cache them in files without extensions and with random filenames. In this example I will show how to retrieve them using Firefox on the Mac, but I assume if you can find the same folders then the procedure should be much the same on all platforms.
First open the page with the video you want in Firefox, let it load completely, you don't have to play it, just so long as it's loaded and cached. Now look for your Firefox Cache folder - on the Mac it lives in ~/Library/Caches/Firefox/Profiles/ - at this point there should be a profile folder with a random name, something like '51tqw9jg.default' - the Cache folder is in there. Open the Cache folder and you should see several files with random filenames - look for one that is about 3MB or more in size (depending on the length of the video you're viewing on YouTube), copy this out to the Desktop or where ever and rename it something.flv (choose your own filename, but the extension must be .flv). This is your Flash Video file.
Either play it like it is, or convert it to a more portable format such as Quicktime. On the Mac I can import .flv files directly to Quicktime because I have installed the excellent Perian, "The Swiss Army Knife of Quicktime Components", after which I can save as a Quicktime .mov file without any trouble (Saving may require Quicktime Pro, can't remember if that option is there for standard Quicktime).
On other platforms you can use VLC, although it's not perfect (it creates green blobs for the first few seconds of video, no idea how to prevent this) but it works. Here's the procedure:
One thing I'm not sure about is file sizes that get cached - I've had this strange thing where no file in the cache seems to grow beyond about 48MB, even though the video in the browser may be larger, I have no idea where the rest of the file lives (maybe in main memory?). This could be a Firefox limitation for cached files, altering the size of the cache in Firefox preferences makes no difference.
Feel free to send me or Ian any feedback on this page!