Not usually one to plug stuff - but the Wayback Machine browser extension is very useful for "This is down, would you like to see a snapshot" (or "This is up now for now, would you like it snap-shotted"... or my favourite "I can't believe they put this up, let me ensure this is preserved for evidence if somebody decides to re-write history")

I use it, but I've been considering uninstalling it due to frequent false positives where I get redirected to the archive when the original is not actually down.