You can use the MacOS Network settings to connect to most types of VPN (hit "+" and fill in the forms to add a new connection). For free, open-source clients, Tunnelblick is very common.
From the known issues page:
"If you are running on OS X 10.6.8 or higher and using OpenVPN 2.3.4 or higher and using a TUN device, the default Tunnelblick setting to "Load Tun Automatically" (on the "Advanced" settings window) will avoid this problem by not loading the tun kext — OS X's built-in "utun" device will be used instead of a "tun" device."