In order to put it on a 'central' storage that can be seen by client box (like a Squeezebox, PS3, Xbox, new Tivo, etc), that storage needs to be smart. It has to run a DLNA server or iTunes server or similar.
If you leave your iMac on, then IT can be the server. With an Airport Express, it really is easy and cheap. And the Airport even supports optical output. Any other solutions will require you to buy some sort of Network Attached Storage (NAS) w/ DLNA and a client box.
Any particular reason you don't want the iMac on? Alternately, if you are trying to store the files on an external HDD, you can always point your iTunes to that particular drive and let it stream to the Airport Express that way. Then you don't have to use up the iMac's HDD space.
