Mediatap is an application that turns your Mac into a music server. It was designed to work in tandem with Audiotap, an iOS app that connects to Mediatap over Wi-Fi or 3G to play your iTunes music. Mediatap is free, but you won't be able to stream music if you haven't bought the $5.99 iOS app.
The initial setup is rather easy. Most of the settings are adjusted by the application itself. Perhaps the only thing that you will have to adjust yourself is your router. In my case, the application wasn't able to contact the server on my Mac from outside my network. To be able to do this, I would have to get into port forwarding in my router's setting.
By default, Mediatap uses port 48848 and Bonjour to connect to other devices, but you can set your own port. Some of the other settings allow you to set a password so that access to your music is restricted and to add new folders with content that you want to share.
Mediatap is also available for Windows, which can come in handy if you want to stream someone else's music to your iOS device.
Audiotap has very good reviews on the iOS App Store, which means that it's users are pretty happy about the way it works.