Look what I found today:
It's an old DE-620 Ethernet adapter that was manufactured by D-Link. This used to be the only way I could get onto an Ethernet network with my Compaq Contura 4/25cx.
It has a parallel port connector on one end (largely obsolete today) for connection to a host PC and a 10BASE2 (thin coaxial, very obsolete) and 10BASE-T (UTP, common today) connectors on the other. This device had wonderful support on the old 2.0.x series of Linux kernels.
I can't recall what kind of bandwidth I could get with it, but I believe it was several orders magnitude better than a serial modem connection (especially since Contura's UARTs could only manage 9600 bit/s), but slower than the theoretical 10 Mbit/s for Ethernet.
Another curiosity was that it required external power through a power brick that's rated at 12 V and 500 mA (parallel port didn't provide power to devices). Today I guess I could almost run a complete EeePC with those six watts of power.