Let’s say there’s a YouTube video clip I really like but am concerned it will eventually be pulled for some reason. Without intending to repost or redistribute it, how might I go about copying said clip so that even if it eventually disappears, I have my own copy? Do I need some special software or application?
Thanks! 
Some clips have a “download” button. If that button does not appear, then the following section from the “Terms and Conditions” would seem to apply:
In other words, if that button does not appear, then you’re not allowed to download it, even if you find some software which might accomplish that.
Sometimes keepvid.com works well.
There’s a Firefox extension called “youtube video downloader” or something. Works good.
Firefox -> DownloadHelper.
Download Helper works great. There are others which also work.
The most rudimentary way to save a YouTube video is to let it load, then go to your browser’s cache and copy it from there. This is in the .flv format (Flash video), which can be played by VLC or other players. The easiest way to figure out which cryptic cache object it is is to clear your cache before going to the video’s YouTube page. You’ll have to rename the object to something meaningful and add the .flv extension. But Download Helper makes the process much easier and it can get the video in mp4 format, which is more widely playable.