As per subject. To give you an idea of what type of devices and client OSes.
Servers/network devices: Windows, Linux servers; Juniper, Cisco, Checkpoint Routers/Firewalls
Clients: Windows, Linux, iOS
We currently use OpenVPN to connect geo-distributed offices and datacenters via Linux "gateway" servers. However, OpenVPN does not fit well (or at all) with Juniper, Checkpoint and iOS.