Good filter design software for Mac OS X/Unix?
MATLAB is a pretty good place to start for filter simulation and design.  There is a filter design toolbox that is pretty useful. However it does come at a cost.

For sensor applications Butterworth filters are generally better as they have a maximum flat passband (at the expense of phase response and roll off).  That means that your signal amplitude will be flat throughout the frequency range.

Stay away from implementing a Sallen-Key topology active filter, it is very difficult to get all the components to be matched and maintain good accuracy, try doing a Monte Carlo simulation on a Sallen-Key circuit to get a better understanding.

Switched capacitor filters are good for steep roll-off and these are available in Butterworth filters.  These do need a single pole before and after them to remove an aliasing due to the clock signal of the switching which occurs anywhere from 50 - 100 times greater than your cut-off.

Alternatively use a simple single pole RC filter (active or passive) and feed into a higher speed ADC and then you can use digital signal processing on an embedded platform or PC to perform decimation and analysis.  This shifts cost and complexity from analog components to software and processing requirements.

Most importantly - ensure you comply with the Nyquist criteria and that you are sampling at at least twice the highest frequency, in practice, this means sample at four - 10 times your highest frequency, to allow for filter roll off well below your ADC resolution level at the Nyquist rate.