Need to design a filter in a jiffy, but don't want to go back to old textbooks for references. Then this is the app for you, where it offers a list of circuit filters from low pass to high pass, from bilinear to bi-quadratic filters, from passive to active filters, this provides an easy guide to quickly select components and visualization of frequency responses by offering a bode plot of the magnitude and phase response. The app also offers the system's transfer function based on the selected components in real time.
Not sure of a system's response and want to analyze the bode plot on your phone, the app offers a bode plot for a user defined transfer function.

Have fun and happy tinkering