I need to implement a simple lowpass filter for a data acquisition circuit (using stretch conductive fabric as a resistive sensor). Various sources have recommended that the best way to go about this is to use filter design software.

I haven't found a filter design tool (calculator) that I like yet. The online ones I've found have been buggy/obnoxious. The best solution would be one with a nice GUI that works with OSX. Alternatively a UNIX/Linux solution could work. Windows is a no-go for me.

Here's an online one that is tolerable, but I'd like to do better: http://www-users.cs.york.ac.uk/~fisher/lcfilter/

I found this list: http://www.circuitsage.com/filter.html

EDIT: I'm using this one right now, specifically for active filters (which are more applicable for sensors as they are more stable with low frequencies).