2 foot Ethernet Cat 5E Crossover cables. The transmit (TX) and receive (RX) wires are swapped on these cables. This allows the cable to be connected to two computers for data sharing purposes without using a switch or hub.
This type of cable is also used to connect older switches and hubs to each other. In the older equipment there is one RJ45 port that is called an MDI-X port used specifically for this purpose.