Playing your music over the internet
TwonkyVision
My NAS uses TwonkyVision as a UPnP server.
TwonkyVision also offers a web interface (http://IP_Nas:9000/webbrowse)
If you use port forwarding on your router, you can browse and play your music over the internet using this interface.
Windows 7
A new feature is too access your media at home over the internet.
You need a Windows Live ID to do so.
A setup guide can be found here: http://www.neowin.net/news/main/09/03/12/windows-media-player-internet-access-to-home-media
Vista
Vista offers the option to remotely access your media using VPN (Virtual Private Network).
A virtual private network (VPN) is a computer network that is implemented in an additional software layer (overlay) on top of an existing larger network for the purpose of creating a private scope of computer communications or providing a secure extension of a private network into an insecure network such as the Internet.
http://en.wikipedia.org/wiki/Virtual_private_network
A setup guide can be found here: http://theillustratednetwork.mvps.org/Vista/PPTP/PPTPVPN.html