There are a number of online tools though they are often limited. Here is a list of online and free desktop tools
Active filter web based tools
Active filter desktop design tools
- FilterLab by Microchip
- FilterFree from nuHerts
- FilterCAD3 from Linear Technologies (Now Analog Devices) Though official links to original sources of it seem to have vanished.
- FilterPro by TI
- FilterWiz Lite by Schematica
Passive filters
- AADE Filter Design tool by KE5FX (Ham radio)
Simulation
I would always recommend simulating the software generated filter with real life simulation models of the components you intend to use for your implementation.
Use a different simulation tool that the generation tool to limit the possibility of software bugs effecting your design.
If you are not dealing with purely sinusoidal signals remember to consider the step and impulse response.