GrooveTube is a mashup of several APIs. It gives you an ITunes style desktop music player, but without having to have any of the music on your desktop.
Music is organised by artists, albums and songs in the same way as any other desktop music player. Just type in a new artist and it's as though you instantly have all their albums available to listen to.
GrooveTube will first search your Music directory for the given song and play it if available. But if the song isn't available it will find it on Youtube and play it anyway.
As with any other desktop music player you can create your own playlists.
Groovetube also comes with a number of built in plugins. Such as artist bios, upcoming gigs and similar artist recommendations for the chosen artist, and lyrics, tabs, etc for the chosen song. And with many more plugins on the way.
This is great! The perfect combination of an iTunes style player and Youtube. It's like I have the full back catalog of all my favourite bands on my laptop. It could do with a help file, but even so, it's all pretty straightforward.