Finetune is a music discovery service that launched in late 2006 and is focused on letting its users build sharable playlists of popular music. You're able to browse and search through a fairly large directory of popular artists and pick out full versions of their songs based on 30-second previews. There's also a really simple music discovery system that automatically builds playlists for you based on your artist picks.
In addition to its Web-based player, Finetune also has a gorgeous application for Adobe Air that both Mac and PC users can run on their machines. It lets you listen to both your Finetune and iTunes playlists, as well as search by name for artists you're into and get a custom radio station of their work.
Winner: Finetune (Finetune.com)