for Mac users: I tried Allegro too and it did not work. Right now I am using Elgato EyeConnect 1.1 and it works really well, but, as the Twonky one, is a commercial application.
I tried to install the port of gstreamer from DarwinPorts, but had no luck in that. I'll try if Fink has it. Or I'll need to compile it on my own.

UPDATE: tried also NiceCast and again no luck.
I finally found a free Media Server that works great: the Philips Media Manager. It's closed source, but at least it's free.

Here is the link: <http://www.streamium.philips.com/>

Choose your language and then click on "Philips Media Manager".