Just googling for "free IPSec VPN" or "free L2tp VPN" will get you a bunch of hits.
I'm not 100% sure I'd trust a free VPN service, of course. If you're not paying for something, you're the product.![]()
I've never setup a VPN, but OpenVPN also has an easy to use client interface for Mac (Tunnelblick).
Apparently, Tunnelblick can also setup the Server side, if you're comfortable with a command-line.
OS X Server also has a VPN service.