An RSS reader of sorts. Distill your favorite music blogs to their free downloads only. Listen. Download. Automagically add new songs to iTunes. In the month I've been using Peel, I've collected a nice assortment of about 120 songs (there were hundreds more that I didn't download). Check it at least once a week, discover new artists, and get a few free songs from some of your favorites. You can also use Peel as a browser for actually reading the music blogs.
From the site: "Every week, artists, DJs, music tv networks, newspapers, magazines, retailers, and bloggers recommend hundreds of their favorite new songs. Critical Metrics keeps track of recommendations and playlists across all media so you can easily find, try, and buy the best new music." I like the find and try part. Not much on the buying part, since most of my music purchases come from eMusic. Critical Metrics is a great way to listen to some of the most-discussed music on the web. It uses its own player to stream playlists based on the recommendations of over 80 online music sites.
I'm still using MusicIP most everyday to create nice mixes from my own collection, which is growing more eclectic thanks to Peel (see above); though, the Filter is a more aesthetically pleasing alternative, its mixes are not as good for some reason.