Because it was so easy, here a little HOWTO describing the simplest OpenVPN setup: connect two servers with a virtual, encrypted ethernet cable:
/etc/openvpn/cable.conf:
local [IP1]
remote [IP2]
port 1194
proto udp
dev tap
secret cable.key
keepalive 10 120
comp-lzo
persist-key
persist-tun
status cable-status.log
verb 5
# openvpn –genkey –secret /etc/openvpn/cable.key
# /etc/init.d/openvpn start cable