Looking to hoard files yourself? Here are some suggestions!
For Youtube and online video in general, youtube-dl is one of the best tools out there. It by default grabs the most high quality version of the video it can find, and combined with ffmpeg or avconv it can go a step further and mux together the separately-stored best audio and video. There are also options to download metadata such as descriptions and thumbnails, which are a must for preservation!
At least for Youtube it's not able to grab the original video file, because Youtube transcodes the original file into all manner of different formats before displaying it to the public. This isn't preferrable but often times it's the best that can be done.
wget is a powerful tool for grabbing resources from the web. It can be used to keep an up-to-date mirror of sites, download everything in a directory with one command, and more. This is a must if you're looking to download any amount of the stuff here.
A quick crash course:
wget -m -np -A=webm,description http://www.badcafe.org/videos/channels/OSD310/
-m
Mirror everything in this directory, as far down as it will go.
-np
Please don't ascend into parent directories and download everything on the site. Please.
-A=webm,description
Only download files with the extensions .webm and .description.
Guides in increasing order of complexity can be found here, here, and here.