Or you could use LimeWire which allows you to specify a set of IPs that will be the only ones allowed to upload/download/connect.

Denying access to router4.limewire.com:6346, connect1-4.gnutellanet.com:6346 and all the other hostcaches will also effectively keep LimeWire clients from ever reaching outside, even if somebody of your staff overrides the host filter.

LimeWire will run on any plattform and you should check out LimeWire's peer server at www.limewire.com too.