Online references, utilities, calculators etc.
Cable resistance/rating calculator Wire sizes AWG, mm2 sizes, resistances etc.
Ohm's Law Tools
Voltage - Current - Resistance - Power Calculator
Resistance - Frequency - Capacitance Calculator
Capacitance - Frequency - Inductance Calculator
dBm - Voltage - Impedance Calculator
Zener Voltage Regulator Calculator
PCB Track width calculator Maximum current, temperature rise above ambient, cross-section etc.
isoStack Tool for designing layer stackups featuring Isola products, Impedance calculator for SE and DIFF microstrip/stripline configurations
Impedance Calculator Tool for calculating the impedance of high frequency printed circuit boards. Supported methods: Surface Microstrip, Coated Microstrip, Edge-Coupled Surface Microstrip and Edge-Coupled Stripline (symmetrical)
saturn pcb toolkit is a very complete calculator for via, traces, diff pairs impedance, resistance, current capacity etc  PCBA and PCB manufacturer
Resistor CAD 2.0 by Terry Harris from Vader Systems
IN3OTD series and parallel resistance calculator
ResOptimzer (WIN32) from EEVBlog forum member jahonen
resistor calculator from jansson
RF Design utilities
AADE Filter Design V4.5 Design and analyse passive filters in both the frequency and time domain.
RFSim99 by Stewart Hyde RF simulation with S-parameters. Not only for filters.
RFSim99 is freeware software that helps with impedance matching and can simulate RF circuits. Plots Amplitude and phase response, different display styles including Smith chart. Uses one and two-port Scattering parameters. You can for example import an S-parameter file of your filter from your network analyser and have the software calculate matching. Requires Windows 95, 98, NT or 2000 (WinXP virtual machine runs it just fine)
RF Calculators A series of RF Calculators optimized for Desktops, Tablets and Mobiles - Attenuator Calculators, Unit Conversions, Skin Depth Calculator, Link Budget Calculator and many more.
Microstrip Patch Antenna Calculator This calculator calculates the length and width of a patch antenna based on the frequency, dielectric constant of the dielectric and the height of the thickness of the patch antenna substrate.
Skin Depth Calculator Enter the frequency and the characteristics of the material to calculate the skin depth.
Power Handling & Impedance Calculator for PCB tracks on Isola dielectic materials, insertion losses