Seedio is a new app that lets you play local iPhone music across multiple devices. The app uses a Wi-Fi network to beam the track to other iPhones in your vicinity and synchronizes the playback. To use the developer's words, the app emulates "one perfectly synchronized loudspeaker" across multiple devices. Each device needs to have the app installed for the streaming to work, but there's no need for all the iPhones to have the track locally for playback to work. The app also supports the synchronous playback of YouTube clips in audio-only mode.

It's a pretty neat solution — we created a personal hotspot using an iPhone 4S and shared a track to another iPhone and an iPad (running in iPhone-compatibility mode — Seedio isn't a universal app), and everything worked flawlessly. Seedio says that you can synchronize playback across five devices using this ad-hoc solution, but the capacity over a regular Wi-Fi network would be much larger. Unfortunately, we don't have enough iOS devices laying around to push the app to its limits. Seedio is available as a free download for now — the developer plans to charge for the seeding app in the future — so you'd better plan your iPhone-powered party soon.

Update: one of the developers of the app just reached out in comments to let us know that the theoretical connection limit for the app is 150. That's a lot of phones.