I make it a routine practice of downloading useful content (e.g. scanned magazines, game manuals, e-books, images, etc.) as soon as I find it. Thus if/when the original disappears, I still have a personal copy that will remain forever accessible.
Yes, I do that too -- and when I grabbed these and uploaded them to my dropbox, I noticed that I had already snagged most of them about 2 years ago. Heh.
I'm finding it's useful to screenshot statements on social media and stuff made by controversial public figures. The Coleco Chameleon clusterbug was pretty well documented by other people doing the same.