LineCalc – calculator for coax cable
Electronic TB (Electronic Toolbox Pro) – includes a multitude of handy electronics design aids – one of my favorites.
uWave Calc (from Agilent Technologies) – a few handy microwave-related calculators.
E Formulas – includes several common electronics calculators.
jEMLab – EM boundary conditions, polarization, incident plane waves and spherical charges calculator.
Circuit Lab – a schematic capture and analysis app.
dB Calc – calculate or convert various quantities in dB.
EE Toolkit– another well-done electronics component and circuits reference app.Digi-Key – has their on-line catalog available.
Buyers’ Guide – ITEM EMC Buyers’ Guide includes a host of EMC-related companies and services.
Directives – is a listing of all the EU directives with links to the actual directive text.
For those dealing with environmental shake tests, there’s Vibration. Although I don’t recommend it, you can bolt down your iPhone or iPad to the shake table and record the vibration profile of a product. It uses the built-in accelerometer.
iSeismometer is similar, but simulates a three axis (3 pen) seismometer. Pretty cool!
If you’re into audio analysis for home theater or wishing to analyze room acoustics, I recommend Audio Tools – pricy in-app purchases, but a rather complete acoustics analysis system.