File transfers. I'm behind two NAT routers which supports UPNP. I would really like direct - fast - file-transfers. File-transfers are dead slow otherwise, and often does not complete.
Of course I could set up the NAT routers manually, but I only have access to one of them...
This would also be incredibly useful for webcam port forwarding in MSN and/or Yahoo! protocols. I run Kopete on multiple PCs on my LAN and it would be nice if Kopete could request one of the valid ports via UPnP and then specify that port to the remote side for the connection so as to avoid conflicts.