Streacom's DB4 Fanless Aluminium Chassis is impressively small at 260x260x270mm, which limits your board choice to mini-ITX and your PSU choices are even slimmer as they need to be both tiny and passively cooler. The chassis is completely fanless and acts as the CPU heatsink, there are integral heatpipes connected to the case and a TIM pad which [H]ard|OCP recommends leaving in the packaging in preference of good thermal paste. The case will sell for $300 and the PSU for just over $200, making this less attractive for some. On the other hand it is very unique looking and provides exceptional passive cooling, if you set it up correctly. Take a look at the full review here.
"This case review is for all of you mini-ITX system builders out there that may be looking for something a bit different than the normal case....that looks like a computer case. The Streacom DB4 fanless chassis may be exactly the HTPC case that will grace your living room....with grace."