Bookmarklet for HQ Videos on YouTubeYouTube sometimes gives you an HQ button to switch to a version with higher quality. But not all videos have this button. You can add "&fmt=18" to the URL and most of the time the video is better than the default one.
Or you can bookmark (or better drag and drop it on the bookmark toolbar) the following bookmarklet: YouTube-HQ
This was done quick and dirty and may stop working tomorrow. It depends on the current HTML code on the page.
Just watch a video on YouTube and then select the bookmarklet. The page gets reloaded with "&fmt=18" attached to the URL.