VPN to Remote Offices/Companies

We have an application in-house that we would like remote users to have access. These remote users are not part of our network/company – they are separate companies with a few users that will need access to this application. We would like to be ableto have these remote offices connect to us via the Internet. These remote offices have various network topologies and firewalls.

Is there a VPN product that I can use to connect these offices to ours? Or is there a different solution that I am not aware of?