One of the things that's always bothered us about YouTube is that there's no way to search for videos without stopping playback of the video you're currently watching (unless you open a new tab or browser window). MSN Video has had a feature for a while that lets you search for videos and create a video playlist without interrupting any video you happen to be watching at the time, but if you want to browse YouTube's library in this way you'll need to look to a third party solution like YouTube Fast Search.
YouTube Fast Search is a nifty web site that lets you play any video you can find on YouTube. While one video is playing, you can search for additional videos and drag and drop them to a playlist window at the bottom of the screen. When your first video is done playing, the next video in your playlist will start. Easy as pie, and something YouTube really should implement on its own site.