Clean DC power design for very high precision circuit
Usually if you need clean power, you need to follow up the SMPS with a linear regulator. You also have to deal with the noise created by the supply, which can be non-trivial. If you really need microvolt noise levels, a completely linear supply might be a good choice rather than SMPS, (or even use SLA batteries and an ultra-low-noise regulator). 

On the other hand, getting 1mV to 10mV p-p noise over a 0.1Hz to 1MHz bandwidth is not that difficult with a SMPS + LDO combination, but care has to be taken in how the SMPS conducted and capacitively coupled noise is dealt with. 

Do not underestimate the noise the supply itself can create- I found that the brick supply for a Tek scope can cause 20mV false spikes in the display if the plastic case is placed near a grounded metal object. That's the supply that the oscilloscope comes with! There are transistors (and possibly entire heat sinks) flapping around by hundreds of volts and switching fast at tens or hundreds of kHz.