When a problem comes up often enough, I look around and see if someone's made a calculator to automate it. Sometimes, I find one, and put it in the links below. When I can't find one, I create my own:

- Voltage Dividers
- Thevenin Equivalent
- Adjustable Resistor Range
- Component of Desired Temperature Coefficient
- LC Resonance: Solve for L, C, R, Freq, Q
- XMEGA USART Baud
- Power Supply Hold-up Time
- Coilcraft SPICE Model Converter
- Convergents (approximations for turns ratios, PLL dividers, etc.)
- Thermistor Data and Functions

- Serge Stroobandt, ON4AA, RF Inductance Calculator for Single-Layer Helical Round-Wire Coils, quite possibly the best damn solenoid inductance/impedance calculator, ever.
- Bob's Electron Bunker, Multi-Layer Coil Inductance Calculator (possibly more accurate than conventional formulas, but still a long way from a capacitance and eddy current-aware calculator; which alas, might never exist for such configurations)
- Chemandy Electronics, Radio Frequency and Microwave JavaScript Calculator Index, covers many typical structures (microstrip, differential stripline, etc.), and other common electronic and RF calculations. (I suspect the numerical accuracy is better than most. Most calculators use the same old piecewise equations, which produces absurd results for parameters outside the approximation range, which is frequently omitted. These calculators solve the Elliptical and Bessel functions as needed!)
- Sourceforge.net, CGI-Wcalc, an impedance calculator toolbox
- OKAWA Electric Design, Filter Design and Analysis (includes 3rd order Sallen-Key and MFB types)
- Jackson Research, Double Tuned Filter, Capacitor Loaded & Capacitor Coupled
- RF Tools, LC Filter Design Tool
- Daycounter, Inc., Electronics Calculators
- MicroModeler, MicroModeler DSP, a web based IDE (non-commercial, limited version)
- Texas Instruments (formerly National Semiconductor), SNVA538, Input Filter Design for Switching Power Supplies
- Jansson.us, Resistor Value and Ratio Calculator
- Kirill Kryukov, Resistor Network Finder (even more powerful than Jansson's!)
- Kirill Kryukov, Resistor Network Solver
- Dr. Heinz Schmidt-Walter, et al., Design of Switch Mode Power Supplies
- Lazar Rozenblat, smps.us, PCB Trace Width Calculator and Equations (includes the heat-dissipating effect of ground planes in multilayer construction)
- Lorusso Compliance Services, LLC, IEC 60950 Creepage and Clearance Calculator