Filter calculator (Resistors in kΩ, Capacitors in nF, Frequencies in kHz):

Select a function to get the corresponding resonance (not complete, go here (p.46), for more info):

Function	Order	Ripple
Bessel Butterworth Chebychev

When designing a filter, you have two mains things to consider: what function do you need, and how you will implement it. Here are the most commons choices:

Functions
Family	Name	Description	Use
Polynomial	1st order	The basic RC filter	Useful for debouncing and smoothing signals
	2nd order	The most common filter	Generic filter, see variants below
	Butterworth	Quite selective filter, really good magnitude response (sharp and straight)	Low distortion and selective application, but take care of the phase!
	Bessel	Not really selective, smooth phase transition	Really good in audio path
	Chebychev	Really selective, but ripple in the pass band (see Chebychev type 2 for a flat response)	Not good for audio, but can be used for signal processing.
Elliptic	Cauer	More selective than Chebychev, but the attenuation is limited	?

Topologies
Name	Description	Use
RC	The basic RC filter	Useful for debouncing and smoothing signals
LC	Simple 2nd order filter, with infinite resonance (but you need an inductor)	Mostly use in power application, as it has no loss (in theory)
Sallen-Key	The most common 2nd order topology	Can implement any 2nd order function, and it's quite controllable: the resonance depend of the gain, and the cutoff frequency of two resistors
Rausch (MFB)	No real benefits compared to the Sallen-Key topology, less component, but harder to tune	Can implement any 2nd order function
Universal cell	Really easy to use topology, but need a bit more components	Quickly design filter, funny to fiddle with, useful when you need different types of I/O