Online calculators are nothing new, but Calculatoredge takes the concept to a new level. On this site, you’ll find more than 200 different calculators in 12 different categories including mechanical engineering, electrical engineering, electronic engineering, physics, and more.
Most all of them are interesting and educational; some of them could even prove to be useful. For example, the “line of sight” calculator determines how far you can communicate by line of sight given two antenna heights. It doesn’t take into account effects such as multi-path propagation, but it will give you a figure that you can use as a rule of thumb when figuring out a simplex link.
Another one I found interesting is the “wire parameter” calculator. Input the wire gauge and it will tell you the wire diameter and the current-carrying capacity of the wire. Crank in the length, and it will also tell you the resistance of that length of wire.
There are lots more. Check it out.