Links:

DMA's Electric Motor Calculator

Nice one. I see some Lipo cells and a lot of cell and motor choices. No way to enter your own values, though, so if it's not there, you're out of luck.

Simple Electric Motor Calculator

A nice, simple calculator. Enter motor type or constants, gear ratio, prop pitch and diameter, and you get a list of rpm, power and efficiency for different voltages. Quick and easy for prop or gearbox selection.

Helicopter rotor speed predictor

Choose from predefined motor, heli and cells, input your pinion teeth and calculate the rotor speed.

Software and documents for heli flight

A couple of spreadsheets for calculations for electric helicopters.

LCRC - Thrust Calc

Static thrust calcs from empirical data.

Propeller Thrust Calculator

Pick a prop from the list, select the type of input (RPM or Thrust), provide a value, and this calculator outputs the output (needed RPM or delivered thrust).

Plane Fun WebOCalc

WebOCalc is a little JavaScript program that can help you design your next RC plane. Given the weights of various items and the wing dimensions, it can give you an idea how fast your plane will fly, and how much power it is going to have.

MotorXL

An accurate motor efficiency and thrust calculator by Rod Badcock

Scorpion Calc and head speed calculator

Scorpion calculator for their motors, both propeller and helicopter head speed calculators.

RCadvisor Calculator

RCAdvisor's very comprehensive calculator, with sections for airplanes, motors, battery, esc, propeller, gearbox, location and airfoils.