Chrome/iOS: If you visit many web sites that offer free music downloads, exfm can archive, categorize and socialise those tracks. The Chrome browser extension is pretty handy, but I'd imagine the iPhone version is a little less useful if you don't do a lot of browsing on your phone.
Once the extension is installed, every time you visit a web site that offers free MP3 streaming or downloads, the extension will add those songs to your exfm library, which offers simple navigation using panes. It does not have an option for playlists as such, but you can "note" songs via a starring mechanism.
The webapp also incorporates social networking; you can add other users as friends and view their songs in a stream as well as post songs to Facebook, Twitter and Tumblr. I've also found that exfm is handy to use for podcasts if you don't have a dedicated podcast manager.
exfm is a great if you visit a lot of music blogs and other web sites that offer free music downloads and want to revisit those tracks in the future. It is a free webapp available in the Chrome Web Store, while the free iOS version is available from the iTunes App Store.