For a long time I've had IPSec VPN tunnels established to customer networks so I could do my routine maintenance securely, but one of my configurations is perhaps a little non-standard: one of my customers is my backup MX (email) handler, but I route mail from him to me through the VPN. This comes into play when my primary DSL circuit goes down and I revert to my backup circuit: because my customer's mailserver knows to go through the tunnel, it doesn't know anything about external IP addresses, and I get all my mail more or less immediately.

Tech Tip: Routing a backup email server through a VPN tunnel