Did you know there are such things as rug heaters? Me either! From Warmly Yours, the 1/4" thick rug heaters fit under area rugs like a traditional rug pad. But these pads you plug in, giving you a pleasant case of toasty toes.
The rug heaters come outfitted with a dual temperature control which you can set to either high or low. Having a house full of quarter-sawn oak floors, I'd probably only use it on tiled (concrete-based) areas--which really are the coldest--as I'd worry the direct temperature fluctuation might compromise all that wooden loveliness.
Available in 3'6" x 4'6" (the perfect size for under a desk!), a rug heater can be yours for $199. To order and for more information, visit Warmly Yours.