There is no such thing as fanless cooling inside a typical ATX case. The only option for fanless cooling are things like the Streacom cases, which have huge external heatsinks, and even those struggle with 65W TDP and higher.
mac g4 cube was fanless
The G4 cube never worked flawlessly (one popular mod was "adding a fan"), but moreover it used a sub 30W CPU (maybe 18W?) with an entry 2D graphics (so I agree with faugusztin about being lower than 50/60W tdp).
I always dreamed of a fanless ATX system: I started with a fanless Solo/P150 maybe a tad less than 7 years ago (with a Core 2 Duo E4400 passively cooled by a ducted Thermalright High Riser and a passively cooled Nvidia 7600GT by XFX, powered by a Fortron Zen): what an headache it was, particularly in summertime (so I agree with faugusztin about "there is no such thing as fanless cooling inside a typical ATX case")...
Well, long story short, I still have that case, but now it sports seven (7) fans, and it runs a lot cooler, a lot more reliably, a lot faster (obviously: since then just the HR is still in place inside it, but paired with a couple of PWM fans), and about as quiet as when it was fanless (as overheating electronics often suffers of several whinings, IME).
Among the only few ATX options available, I would point you out to the Nofen CS-series (I like a lot the very pricey model 70), to be paired with the CR-series heatsinks: but personally I would not pair them with "enthusiast-range" parts. Take a look at their offerings on some websites such as quietpc.com, there they starts from something more than 100 euros (CS+CR).
I know that Intel sells 4.5 watt TDP Haswell to laptops.
Intel's New 4.5W Core “Haswell” Chips Blur the Line Between ...www.xbitlabs.com/.../20130723192913_Int ... Core_Has..
Jul 23, 2013 - ... lineup of ultra-low-power chips it has to offer, yet are based on high-performance Haswell micro-architecture. With power consumption of only ...
Intel starts shipping 4.5 w Haswell Core i3 chip for laptops - Geek.comwww.geek.com/.../intel-starts-shipping- ... p-for-la..
Sep 10, 2013 - So what exactly will this new Core i3 offer over existing low-power chips in the ... The chip's Thermal Design Power (TDP) is actually 11.5 watts, ...
Intel readies first fanless Haswell processor for tablets and ...www.phonearena.com
Jul 24, 2013 - Needless to say the 4.5 watts fanless Haswell is destined for tablets and ... of power, compared to the lowest power envelope of 6 watts that the ...
There are ultrabooks and tablets with no fans using Haswell, so if you can have fanless Haswell in an ultrabook slim form factor, using a huge heatsink in ATX should also be possible.
Does anyone know the lowest watt Haswell TDP for the desktop you can buy at newegg? specific model/pricing
in a few months a 14nm Haswell is coming out.