Seeqpod started with the idea that you could look up any song and immediately start it streaming and create playlists. Now Groove Shark has built upon that idea by letting you look up any song, immediately start streaming it and save it to a playlist. They have added more features and introduced a really simple, yet intuitive interface that is similar to Google. My favorite feature is being able to easily preview a song before you play it.
Thanks to Jason from Thunder::Tech for the tip on this entry!
2017 UPDATE: Groove Shark closed so here are some alternatives http://leadinghands.org/blog/streaming-music-services/