Open source VPNs are available, but are totally unsupported by any central responsible party.

FreeS/WAN is probably the leading product in this niche. It runs under Linux. There is fairly detailed documentation that goes along with it, and you can connect it with other IPSec solutions being used elsewhere in the system. The IKE module uses RSA signatures or shared keys only. (Remember, it was mentioned earlier that X.509 certs aren't being used much yet -- there's a bit of a chicken-and-egg situation currently going on.)