doubleTwist (Android) / Remote (iPhone)
Why you need it: For your BBQ you can get fancy and hire a DJ, or you can not and just make a playlist. Or you can get the best of both worlds with either of these two apps. Apple’s Remote app is simple—it allows you to remotely control your iTunes library on your computer. On the Android side, doubleTwist is a full-blown media player which lets you sync your iTunes playlists with your phone. Cooler, though, is the ability to upgrade to AirSync so you can control your the music on your Mac or PC.
Downloand Remote from the Apple App Store, Free
Download doubleTwist from Android Market, Free