As Richard says, as is usually the case, it depends.... :-)

If there are severeal boards connected together within the same enclosure like thru a backplane connector etc then you could possibly use the open drain UART aproach (bus pulled up by a resistor and each device yanks it low) for the physical layer - then you apply a PROTOCOL (ie the "language" the devices uses to speak to each other).

If the boards are spread out I'd probably look at RS485 as the physical layer, either half or full duplex depending on what you're doing. Same thing again, you apply whatever protocol you want and transfer it via RS485.

A protocol can be as easy or as complicated as you want/need to make it. MODBUS is a fairly common master/slave protocol for industrial stuff but it might not suit YOUR needs at all.

CAN is another option, which is a differential bus (like RS485) with a specified protocol but the complexity increases.

/Henrik.