The 112 x 88 mm (4.4" x 3.3") fitlet2 is powered by the 12W Atom x7-E3950 and features one M.2 slot (2260), one SO-DIMM slot (up to 16GB RAM), four USB ports, a mini SD card reader, and two Intel Ethernet ports. $199
The Kaby Lake barebone rocks a Core-based Celeron 3865U and features two SO-DIMM slots (up to 32GB RAM), a card reader, six USB ports, DisplayPort and HDMI ports, as well as two Intel Ethernet ports. The DS77U also comes with dust covers to hide the COM ports. $200
ZOTAC's best-selling Apollo Lake barebone comes with the 6W Celeron N3450 (4 cores up to 2.2GHz), the long-awaited HDMI 2.0 port, dual RAM slots, and dual Ethernet. 5-year warranty is a nice touch too. $170
The fastest 35W desktop processor available is a 4-core / 8-thread wonder. Intel's i7-7700T also features 8M cache and is capable of turbo-ing up to 3.80 GHz. The small thermal enveloppe makes it a perfect choice for passive cooling. $352
AMD's answer is a much more humble - and affordable - 35W APU (4 cores up to 3.80 GHz) packing solid integrated graphics (8 GPU cores / Radeon R7 Series). $160
The last dinosaur. Silverstone HE02 is a one kilogramm monstrosity (and we mean that in the best possible way). The very last official - and performing - fanless CPU cooler. $77
While technically not a fanless heat sink, Phanteks PH-TC14PE is an absolute beast and will handle 35W processors passively... in the following cases. $92
The smallest (16 x 10 x 19 ") cutest, and fully ATX-compatible chassis from Fractal Design can fit CPU coolers up to a towering 160 mm. Use it without Fractal's ModuVent covers. $77
Thermaltake Core P1 offers the best convection since, well, there is no case. The open-frame structure is also less dusty than you would think. No fan means no dirt being pushed into the system. $100
Seasonic's luxurious 600W fanless power supply is a new milestone in personal computing. Extremely stable and reliable, the fully modular unit comes with a 12-year warranty. Highly expensive and highly recommended. $200
Silent (and blazing fast) storage is the icing on the cake of all passive builds. Samsung's SSDs are our storage of choice for years and the current 860 EVO series offers the best bang for your buck - by far. 1 TB ($300) 2 TB ($650) 4 TB ($1400)
