Adafruit 2.1mm female/male barrel jack extension cable [ADA327]
This product is a 3-in-1 cable. By itself, it can act as an extension cord for any 5.5/2.1mm DC power cable. The heavy 24 AWG wires can handle up to 5A.
You can also cut it half to make two barrel cables (one plug and one jack). This will allow you to connect a classic 'wall wart' to a breadboard or PCB without having to solder in a bulky power jack. Or you can use the plug half to connect some batteries to something with a 2.1mm jack.
The cable is long too at 5 feet / 1.5 meters.
Note: As of Wednesday, October 7th, 2015 we are selling a revised version of this cable with a 24 AWG