With so many fine small form factor motherboards being available these days, the SFF market is becoming more and more attractive to a large share of the computer enthusiast market. AnandTech just wrote a small form factor buyer's guide, you can check it out over here. The article discusses budget SFF builds, file server builds and gaming SFFs.
Early last year I wrote a guide featuring nettops, small form factor computers that were useful for the most basic computing tasks. These computers are now all but dead, having been replaced by the explosion of tablets. However, more powerful small form factor systems remain a viable option for a desktop computing solution. Intel's current Ivy Bridge-based CPUs have very low TDPs—even some quad-core SKUs have TDPs of 55W or less under full, sustained load. And AMD's current Trinity APUs pack a quad-core CPU and discrete-level GPU into a 100W thermal envelope. Both Intel and AMD solutions will typically produce far less heat than that, too, considering most people do not put their computers under 100% load for extended periods of time, and these chips idle at low power consumption levels. Furthermore, any PSU worth its salt features 80% efficiency or better, and DDR3 memory pulls 1.5V or less. We've come a long way since 2006!