i was curious about these units several months ago, so i went out and bought a minikeg of heineken on sale at the local supermarket for $15.99 for testing purposes....
i've learned that beertender is a trademark for the kegging system, which was developed by heineken and krups but other manufacturers make beertender-compatable machines as well. the systems have been available in europe for a few years and several brands are packaged in beertender kegs, but so far in the states heineken is the only brand available.
the heineken kegs come with a tap/spout assembly that snaps onto the valve on the top of the keg, that is intended to be placed in a refrigerator for cooling. the beertender units don't use this tap/spout, instead they use a disposable, single-use tapping line that fits in the beertender's tap assembly. a few spares come with the machines but you have to order more when they're used up.... the machine's tap isn't an actual faucet, it's just a holder/actuator for the valve in the disposable tapping line
the beertender keg itself is made from steel and is not reusable. it is pre-pressurized so no co2 cartridges are used in the machines. as a result, the machines can't dispense from "standard" 5l mini kegs, although the kegs will physically fit into the machines. as far as i know, there are no adapters available...