Extension.fm is a Chrome extension that catalogs the mp3 files from the sites you visit into a web-based music library.  When you visit a site, the extension adds the site and its mp3 links to your library.  As the site is updated, the new mp3s are automatically added to your library.

Extension.fm provides an interesting way to discover music, as you have a constantly growing stream of music.  There’s also an iTunes like UI, which lets you access your library by song, by artist, or by source site.

By mapping song metadata to mp3 links across the web, Extension.fm is building a music catalog around which it can enable other features, like playlist sharing or a social recommendation service like M-Flow in the UK.  Of course, the catalog is limited to mp3s available on the web, but there aren’t any licensing roadblocks - since the sites hosting the mp3s are responsible for licesning, paying SoundExchange, etc.  This dynamic may change if Extension.fm’s usage grows significantly, particularly since the service won’t necessarily drive traffic to the underlying sites they way HypeMachine does (the source site of a track, while visible, isn’t really central to the product). 

Regardless, it’ll definitely be interesting to watch the product develop.  Dan Kantor, the founder, previously founded music-streaming service Streampad.  And the company recently raised a first round of funding from music-loving VC Bijan Sabet (Spark Capital) along with Betaworks and Founders Collective.

A nice addition to the NYC music-tech scene.