Android: Want to create a big speaker system, but don't have any actual speakers? AmpMe is an app that allows you to chain together phones and Bluetooth speakers into one big synchronised sound system.
Once AmpMe is installed on your phone and a few others, the host simply needs to start playing music, then everyone else can join in to turn their phones into speakers as well. Sadly, it only works with SoundCloud, so there's not a ton of options for music, but it's easy enough to find something to listen to.
The end result is a loud, tinny, cacophony of music, but it certainly works well in a pinch. Though, if you don't have any friends, a paper cup works just as well.
AmpMe (Free) [Google Play]