I don't know if you know the Remote app. It's made by Apple and it's free. It allows you to control what's playing in iTunes over WiFi.
Now, in case that doesn't fit your needs. What is it exactly you want to do? You want to play music on your iPhone/iPod that's located on an iMac? Why not sync the songs to your iPhone/iPod? What would be the added value of such an app?
I'm really interested in this, being an app developer that already has a couple of apps that connect to a Mac over WiFi.