Rafirafi launched the pcnet32-driver-for-puredarwin project to produce a kext driver for the PCnet Ethernet network card for PureDarwin/Darwin 9. This is a driver for the "PCnet/PCI II 79C970A" card found in QEMU and VirtualBox for a PureDarwin install.
Grab it while it is hot: https://code.google.com/p/pcnet32-driver-for-puredarwin/