Here's what it looked like when my editor listened to my Spotify using SpotCast.

App.net is garnering lots of attention among the technorati for its innovative business model — a subscription-based, ad-free, microblogging social networking service and an open API for third-party developers. A “real-time social feed,” App.net pledges to do basically what Facebook Connect does, but without being Facebook (see “Dear Mark Zuckerberg“).

Although it only reached its funding goal of $500K less than a month ago, App.net is already powering a number of third-party apps. Now, we’re beginning to see the first music apps roll out on this new platform, the most interesting of which is SpotCast by @jazzychad.

Dubbed the new “Turntable.fm for Spotify,” this Mac app, which runs in lightweight form in the status bar, lets users DJ songs through Spotify while their friends listen to what they are playing in real time, also through Spotify. Both must have SpotCast installed, and the app authenticates users via app.net (join here) or, for those who don’t have $50 to spare, Twitter.

My SpotCast friend chose this song, and I get to listen to it.

Before you get all fired up and install this app, check that you have the minimum requirements for SpotCast: 1) Mac OS 10.6 or higher; 2) Spotify account (requires Facebook if you’re a new Spotify user); 3) Twitter or App.net account; and 4) friends who also have the above. One thing you don’t need is money, because SpotCast is free, and it works just fine with the free version of Spotify.

After you’ve set up your DJ or listening booth (instructions here), you’ll want to launch and log into both SpotCast and Spotify. Now, you’re ready to begin the group listening fun.

If you want to DJ, select “On Air” from your SpotCast menu in the status bar. Then, listen to tracks on Spotify like you normally would… and voilà: you’re a DJ.

Of course, you’re not a DJ if no one is listening to you. it’s like that proverbial “tree falling in the forest” thing. You’ll need to tell your friends you’re going to be playing music, and make sure they have SpotCast, Spotify, and your Twitter or App.net username. Then, they can listen to the tracks you’re spinning by selecting “Listen With…” from their SpotCast menu and adding your username.

To do the same, simply unselect “On Air” and choose “Listen With,” then enter the Twitter or App.net user you want to listen along with.

When we tested SpotCast, we ran into a few hiccups at first, but eventually got the app up and running after as few tries (one of our test Macs repeatedly crashed after we entered a Twitter name, for some reason). If you’re the listener, keep in mind that you may not hear what the DJ is playing at first, but once the song changes once or maybe even a few times, you should be good to go. Once we got it working, my editor and I were able to DJ songs for each other from separate boroughs of New York — pretty cool.

This isn’t the first time we’ve heard of an app being a “Turntable.fm for Spotify.” Soundrop, a Spotify app that runs within the desktop version of Spotify, has been claiming that title of late. Unlike Soundrop, however, which acts as a crowdsourced playlist allowing all users to add their own tracks and vote songs up or down, SpotCast reserves DJ duties for only one person while everyone else listens — just like in real life.

Although SpotCast does enable group listening, we wouldn’t call it the next Turntable.fm, which could be a good thing. This app fills a different need, by allowing one person (probably the control freak of a group of friends — or maybe you could rotate each day) to play music, while the rest listen without having to worry about queueing up songs.

We can imagine many interesting uses for SpotCast, such as letting one person in an office DJ music for everyone else; having a remote friend DJ your next party from a different location if they live too far away; listening to music with a loved one across the country while chatting, and so on. As with the real-life version, though, SpotCast is not worth the trouble unless you trust the DJ.