OpenVPN uses TAP-windows to provide virtual tap device functionality on Windows. Normally you don't need to install TAP-windows separately, as OpenVPN installers include it. The tap-windows driver comes in two flavours: the NDIS 5 driver for Windows XP and NDIS 6 for Windows above. Source code for both tap-windows drivers is available on GitHub.
Installer (NDIS 5) | tap-windows-9.9.2_3.exe | GnuPG Signature |
Installer (NDIS 6) | tap-windows-9.21.1.exe | GnuPG Signature |
Most other operating systems(linux/unix) have virtual tap device functionality in their kernels.