Vincero Designs' Edge16 system comprises both a slim magnetic mount and a purpose-built water bottle with a magnet built in it. The two magnets lock together, providing a powerful bond to keep the bottle securely on your bike. The main advantage is the beyond-tiny weight of 16 grams.
To be honest, I knew the 16 grams was low, but not being an ounce-hoarding roadie, I wasn't quite sure how low. So I did a quick comparison. This Easton bottle holder, which uses every industry's favorite weight-saver--carbon fiber--weighs in at 27 grams and costs 50 bucks. Meanwhile the Campagnolo Record Breaker, another ultralight carbon fiber model that comes close to the Vincero's low weight at 20 grams costs an are-you-friggin-kidding-me $220.
In that light, the Vincero is not only light, it's a pretty solid deal at $47 for the bottle and mount. It is available in both 20 and 24 fl. oz. sizes.
Another advantage that the design might offer is that it seems like it'd be easier to pull your bottle straight off mid-ride than to have to finesse it in and out of a cage. Speaking as someone who just cracked his aluminum cage, I'd also think the small, flat nature of the Edge16 would give it some extra durability.
For a closer look, GearJunkie did a review.