There is really so much happening in this space right now, it is challenging to keep up. While the well known pioneers of music recommendation (like Pandora, LastFM) continue to make headway, new entrants are continuing to redefine how it all works. Extremely interesting developments almost on a daily basis.
Here are two I recently came across that take MP3 blogging to the next level. Peel is an MP3 Blog reader and player rolled into one (actually both of these are). You subscribe to various blogs and easily traverse the music leading you to things you never knew existed. Songbird is a media browser that takes over your iTunes collection and integrates it into the rest of the online music world. Truly amazing. Check em out and have fun! You never know what you are going to find next.