The Antec ISK 100 (now superseded by the ISK 110) HTPC chassis is a worthwhile consideration for HTPC enthusiasts. It measures only 222 x 78 x 212 mm making it small enough to be mounted on the back of a LCD display that has accessible VESA mounting holes.
A small desktop stand and VESA mount adapter (not shown) are included with the case.
UPDATE: I have checked and found my existing Mini-ITX based system with the Gigabyte GA-350N WIN8 board consumes no more than 50 watts from the mains, running off an old inefficient desktop power supply and driving a desktop HDD. I expect the internal power usage to be in the range of 30-35 watts, well within the 80 watts or so that the internal power supply in this chassis will be able to produce.
UPDATE2: I was pleasantly surprised to find there are a considerable range of Mini-ITX boards made, many of which have a CPU socket instead of an embedded chip. Still, the onboard unit was significantly cheaper and has done everything needed of it.