ExtensionFM is a free Chrome add-on that catalogs every free MP3 file you run across and builds a virtual library. It's an amazingly convenient way to discover and catalog new music without waiting for downloads, and may convince me to use Chrome on a regular basis.
Chrome has always seemed like a solution in search of a problem: I've had Firefox installed on my PCs and Mac for years now and it works fine 99 percent of the time. If I need an alternative I can always go with the built-in Internet Explorer (Windows) or Safari (Mac). Chrome may render some pages more quickly, and I like some of its user interface features, but not enough to switch.
But ExtensionFM actually changed how I think of Web browsing, blurring the line between offline and online in a very seductive way. The basic idea is straightforward: install the add-on, and from that point on, any time you run across a page that has a link to a downloadable MP3 file, ExtensionFM will add a permanent link to that MP3 to its library, which looks a lot like iTunes (or just about any other music player). The ExtensionFM library is always accessible from Chrome--just click the icon in the upper-right corner of the browser, and you'll launch a tab with the library. The library itself lets you organize all the MP3 links by source, artist, or album, and you can stream any song on demand or add it to a queue. If you don't like a song, you can delete it from the library.
The experience is a lot like surfing the Web for free MP3s and downloading every single one of them, except without waiting for downloads. I installed it and after about five minutes of surfing I had an on-demand library containing more than 100 songs from music blogs like Spinner and Brooklyn Vegan, as well as from a couple of Seattle bands that have made free MP3s available on their sites. ExtensionFM will continue to feed new links from these pages into its library with no further intervention on my part--every time the Spinner home page is updated with more free MP3s, they'll appear in my library.
The experience isn't perfect. Some of the listings in the library didn't link to a real sound file, and I had to delete them manually. Some listings had wrong or missing data (no artist name, or a title like "Free download"). It doesn't work at all with files that require you to launch a mini-Flash player to play. And the library could get large and cluttered quite quickly. Nonetheless, if you're constantly on the hunt for new music, this is a great way to access large volumes of free music without having to download each file yourself.
It's also the first really great example I've seen of how Google envisions the future of the Web, in which the lines between offline and online blur and the Web browser becomes the only application you need. Sure, there are plenty of Web apps today--I spend a large part of my day in them, including Google's Gmail service. But most Web apps run inside a browser window and disappear as soon as you close that window, and the application itself is responsible for storing data (usually in a back-end database, sometimes in the browser cache). ExtensionFM is a persistent application that runs in conjunction with the browser regardless of where the browser's currently pointed, and it stores only the links to data, which can come from multiple sources around the Web--the data themselves never leave their original spots. It's a subtle but fascinating difference.