Useful for creating filters, debouncing buttons, and isolating power for integrated circuits.

Simple low/high pass RC filters can be constructed with ease using these in combination with other capacitors and resistors.

A filter calculator can be found here: http://sim.okawa-denshi.jp/en/CRlowkeisan.htm

Isolating your ICs and other devices is a good idea, since it can prevent instability and unpredictable effects in noisy environments (like motors or switched power supplies). Here's a good introduction to ripple voltage/current: http://www.seattlerobotics.org/encoder/jun97/basics.html