The Youtubian bookmarklet makes YouTube lighter and simpler by turning the site into a one-page ordeal. It places search results and full videos on one page to avoid reloading, while advanced features like video download links are added to the sidebar.
To activate the Youtubian version of YouTube, just navigate to a video on YouTube and click the bookmarklet — the page will reload with the video, a search box on the right, related videos on the bottom, and download links for Flash, MPEG and mobile versions of the video. If you search for a video using the page's search bar, the results show up below the video, and can be opened on the same page — no reloading or redirecting necessary.
The gain in features comes with the loss of some other features — such as video comments. But the nice part about it is it's not a script that always changes YouTube — you only get the minimalist version when you use the bookmarklet, so you're never locked out of any of YouTube's functionality. You can always go back to regular YouTube by hitting the "Expand" button at the top of the page and using any of those links to get to the home page, your account, or anywhere else.