We currently have a PPTP VPN, but we have a couple of people off site who are on a network which only allows outgoing http/https and ssh. For some reason, it appears that they cannot connect to our VPN because of some issues on the network where they are trying to connect.
I'd imaging being able to run some software on their laptops which sets up a virtual nic on a private IP address, and then forwards all traffic via ssh to a machine on our network.
Our users are running on Windows and on Macs.
Set up an openvpn server on your network, and let them vpn with that. It can work over tcp 443 as a default configuration. Then they can use ssh and everything else to get to your network over the vpn tunnel.