For sites that don't change often (say like chinese dictionary sites), Greasemonkey does wonders. Having to keep up to date with google or youtube's site updates just sucks though. So stuff like videodl.org has popped up.
That's nice and all, however, I like greasemonkey because I put things where I want them (no unnecessary cutting and pasting into videodl). I guess greasemonkey needs something like gpg-signed scripts. So trust an author and just point a greasemonkey loader towards a repository. When a new release appears, you get the new script provided it passed author signature validation.
It seems like the pieces exist: pgp to piggyback on that key infrastructure, just need to try it out and see how it goes. Maybe this would help centralize the development efforts, so we get more complex scripts....