How can noise be good? It seems like (controlled) noise can sometimes be good: dithering in an image, or a softer sound in audio, or white noise alone. Here's a paper on noise improving detectability. My intuition also says that noise in a financial trading algorithm could act as a sort of hedge.

I have few examples — only a vague philosophical instinct. Is there any general-purpose literature or theory on why and how noise can be good? Or other/better examples?
Another example is setting up A/D converters - usually the input is set so that the noise just trips the least significant bit (LSB) of the converter. The noise can also be added manually - this process is also referred to as Dithering. The white papers at various A/D manufacturers can be quite useful e.g. Linear Technologies, Analog Devices and Texas Instruments.

Another term you may wish to look under is Stochastic Resonance.