Windows/Mac/Linux (AIR): JamCloud is a free social music app that lets you "DJ" videos and playlists from YouTube and SoundCloud.
You'll have to create an account at JamCloud before you can download and log in the app, but once installed you can drop into active rooms to see what music, music videos, or other YouTube videos people are sharing and watching. If you like what you see, click "woot", and if you're not a fan, click "meh". Chat with other users in the room using the chatbox in the right corner, share videos you like on Twitter and Facebook, or click to DJ and play videos you want to see.
If you're not feeling social, JamCloud also lets you import your YouTube playlists, search for videos and add them to your library, and just watch web videos from inside the app. JamCloud is free, and comes without the location restrictions that TurnTable.fm has: it's available everywhere.