Lucid Electronics Workbench is very useful utility for calculating resistor, regulator and power values.

# Calculate LED Load Resistor Values: Calculate LED Load Resistor Values Calculate the value of a load resistor for a Light Emitting Diode (LED) based on supply voltage and LED characteristics. This application contains a number of standard LED products to choose from. After the value of the load resistor is calculated, a table of standard resistor values can be displayed so that the closest resistor value may be selected for the intended application.
# Power Supply Component Values: Power Supply Component Values Select from a variety of Power Supply types and enter the transformer voltage and power supply load. This application will calculate the values for diodes, capacitors, output voltage and load current.
# Ohm's Law Calculator: Ohm's Law Calculator This handy function will calculate the classic values of Resistance, Power, Current and Voltage when any two of the values is entered in the calculator.
# Regulator Circuit Component Values: Regulator Circuit Component Values This application contains a number of Adjustable and Fixed Regulator types. Web links to regulator components are provided when available. Case styles and pin outs are displayed. Capacitor values and Resistor values for adjustable regulators are displayed based on desired output voltage and supply voltage. Use the 'Power Supply Component Values' function to calculate the input supply voltage.
# Transistor Circuit Component Values: Transistor Circuit Component Values This application calculates the Bias Resistors, Emitter and Collector Resistors, and the Capacitor values for Input, Output and Emitter Bypass. Actual component values can be displayed, as well as component values based on standard component values. The output, including an applicable schematic, may be viewed or printed.
# 555 Timer Component Values: Transistor Circuit Component Values This application calculates the component values for a timer circuit based on known information: 1) Calculate Frequency and Duty Cycle based on C1, R1 and R2 values; 2) Calculate C1 and R2 values based on Frequency and R2 values; and 3) Calculate R1 and R2 values based on Frequency, Duty Cycle and Frequency tolerance values. Actual component values can be displayed, as well as component values based on standard component values. The output, including an applicable schematic, may be viewed or printed.
# Zener Diode Regulator Component Values: Transistor Circuit Component Values This application calculates the Load Resistor and Filter Capacitor values for a simple Zener Diode regulator circuit based on circuit load and Zener Diode charateristics. The output, including an applicable schematic, may be viewed or printed.