The built-in media player is a UPnP/DLNA client.
At least for the N8x0 somebody already ported MediaTomb UPnP server. Brisa is another server available for N8x0. Porting gmediaserver or ushare shouldn't be hard, too. Or Rygel, as the UPnP subsystem of Maemo5 is based on gupnp.