On PlanetSky supported many types of VPN:
For all connections available - PPTP type of VPN with 2 options
1)Your connection goes completely over satellite, it's good if you dont want any incoming terresterial traffic.
2)Control VPN connections comes over terresterial line, and data flow incoming goes over satellite.
PPTP VPN supported on Windows versions start from Win 95 OSR, and also Linux, FreeBSD.
For corporate accounts:
GRE or(and) IPIP VPN - supported by FreeBSD, Linux, Cisco.
CIPE VPN - supported by Linux.