There are of course other ways of setting up a VPN. Here are a couple of other systems:
PPTP is a Microsoft protocol for VPN. It is supported under Linux, but is known to have serious security issues. I do not describe how to use it here since it is covered by the Linux VPN Masquerade HOWTO.
IP Sec is a different set of protocols from SSH. I don't actually know all that much about it, so if someone wants to help me out with a description, I'd be most appreciative. Again, I do not describe how to use it here since it is covered by the Linux VPN Masquerade HOWTO.
CIPE is a kernel level network encryption system that may be better suited to enterprise setups. You can find out more about it at the CIPE homepage.