Often I have a 3.3V or 5V mixed-signal circuit with a microcontroller running in the 10 MHz range and some analog circuitry, e.g. audio or analog sensor signals, powered from a wall-wart style supply or USB.
So a problem I often come across is how to filter the supply for the analog signals. Shall I use a ferrite bead or a choke? More inductance or less? Or just an RC filter?