JVN Calculator is a spreadsheet which helps with the design of robot mechanisms and drivetrains. With this, you can find the speed of your drivetrain, the current draw from motors, and design rotary and linear mechanisms.

ReCalc is website with various mechanical calculators and references. This calculator is web based and has sharable urls which make it very easy to save and share configurations.

The AMB Robotics calculator is website with various mechanical calculators and associated formulas. This calculator is web based with shareable links and the ability to change units between metric and imperial.

The Spectrum design spreadsheet combines tons of useful resources together for calculating pneumatics, mechanical systems (belts, chains, gears, etc.), electrical, and much more. This is a great spreadsheet to always have open while designing.

This spreadsheet provides a lot of advanced calculations for FRC drivetrains. You can use this to optimize your gear ratios for specific travel distances and current draws.