It's not actually weightless, of course, but the Weightless Headset Phone is less than 1.5 ounces, so you can stay tethered to it and never have to get up to answer the phone or stop what you're doing. It's not just a headset, it's the actual phone with an integrated keypad for dialing out. You can move 100 feet away from the recharging base without losing reception. Fits over one ear so you can keep the headset for your mobile phone over the other ear.
$130 at Hammacher Schlemmer.