Tunes Remote Desktop allows you to control your iPhone or iPad from your Mac. To do this you have to install an app on your iOS device and the desktop client on your Mac. The desktop client is absolutely free, but you will have to pay $0.99 for the iOS app. If you have more than 1 iOS device, you can even control one with the other if you have the app installed on both devices.
Tunes Remote Desktop can let you control the playback of your iOS device's music. It somehow manages to stream your iPods songs' information to the Desktop Client so that you can play any of the songs remotely. You can control the volume and play the next or previous song from your Mac, and the sound will come out of your iOS device. This is an ideal app for when you have your iPhone connected to a dock or audio system and you want to control it from a remote location.
You can protect your iOS devices by using passwords that have to match for them to make a connection. By default, the password is "00000", but you can change that.
I was surprised at how quick and responsive both apps were. My only problem or rather, what I think is missing, is that once a song stops playing, nothing else will happen. You can't set the app to keep playing your songs. Basically, they don't play like if they were part of a playlist. And I don't see an option to configure this.