Ibizaah!: YouTube Video Mixer & Playlist Creator

Ibizaah! is a cool online YouTube video mixer and playlist creator that lets you play and browse YouTube videos at the same time. You can add videos to your playlists and fade sound between songs on the fly. It also has a really cool and addictive air raid siren that goes perfect with any techno, house, or club tracks.

From the page:

I built Ibizaah! because I was annoyed at having to switch between windows when trying to keep the music going at a party. Not that that was a huuuge pain, but big or not, from now on that pain is gone (I feel so relieved.. ahh!) and I hope you will too