[maaku]

Meh, you can do fine by using two one-way ethernet cables (you might have to cut the receive wires yourself), and some tweaked network stack.

[shabble]

The ol' DIY Data-Diode[1]

I've heard of using serial lines/modems with the appropriate tx->rx cut, but I don't know if it would actually work for ethernet (maybe 10BaseT only?)

[1] https://en.wikipedia.org/wiki/Unidirectional_network

[spin]

I'm just speculating here... but I think it could be done PC-to-PC up to 100mbit. IIRC, there's a "link" signal that normally exists -- you'll have to configure both cards to ignore the link signal. (Which, I suppose, would mean that this wouldn't work going to a switch or hub with factory-default firmware.)

Half-duplex, 100mbit, ignore link. I suppose it can be done?....