How to connect 2 servers over a WAN

Does anybody know the easiest way to connect to LANs together. I’ve got 2 seperate networks (each network has 17 client pc’s w/ one server running NT4.0 Server,DNS,WINS,DHCP,Echange)They have seperate ISP’s with seperate IP’s, Internet access, etc…
The networks need to be connected for file sharing purposes and nothing else. Any suggestions would be much appreciated.
Thanks.
-Clueless