Last time Youtube's Video Editor was launched, it didn't work for me – they had something like a light version of Adobe Premiere, IIRC, and apparently discontinued that before I could try it. It probably became live afterwards, but I didn't notice that. However, a few days ago they launched a new version, that actually features some nice editing options: http://www.youtube.com/editor

Here's the announcement at the Youtube blog: http://youtube-global.blogspot.com/2011/03/lights-camera-edit-new-features-for.html