The program MiscEl is a free program designed to handle lots of different electronic calculations.
See here for screen shoots and download:
Some of the supported calculations are:
Ohms law, impendace of capacitor and inductors, complex power
Decibel and neper, both power and voltage including volt/watt for a reference level
Capacitor/inductor charge/discharge with resistive, current and power loads (not all combinations supported)
Calculates RMS, mean, PEEK, DC, FFT for different waveforms
Convert between color codes and numeric values for many types of components.
Color/wavelength of leds and design of circuits to drive leds from DC or AC.
Calculate crc, checksum and modulus for datablocks, can also analyze a datablock for what checksum/crc was used
Calculates Fo for RC, RL, RLC circuits
Design/analyze voltage dividers, can include component tolerance and load.
Analyze and design 32 different 1. order filters configurations