YousableTubeFix changes YouTube from a world of pain to a much nicer experience. The biggest improvements (IMO) are:

* the video window is a reasonable size (not fullscreen, but not too small either)
* any video is trivial to download
* switching to high quality is easy

It’s such a pain to track down the Flash or Firefox cache directory, for those cases when you want to download a video from YouTube… and it’s irritating to use third-party plugins or websites, when it’s clear that the browser is already capable of doing everything it needs to.

So you know, you must install Greasemonkey first, but you should already have done so since Greasemonkey is so damn cool.

RTFA: http://userscripts.org/scripts/show/13333

Removes ads and unwanted sections (configurable), allows downloading and resizing videos, displays all comments on video page, expands the description, can prevent autoplay and autodownload, adds a HD (High Definition) select, etc…