Seafoid is a Mac OS X application for calculating electronics related stuff

Seafoid lets you find out:
* resistors' values from their color bands
* the total resistance of series and parallel connections
* the voltage (U), amperage (I), power (P) and resistance ® in every direction
* dropping resistor values for LEDs
* the thermal resistance for the right heat sink
* information about vacuum tubes (heating, socket, contained systems)
* inductances of air-cored coils

And yes, it is free.