RF (Radio Frequency)
The How To Mechatronics website has a great tutorial on using the NRF24L01. The most crucial aspect is to check the voltage requirements!!! (use the 3.2V not the 5 V)
The Instructables tutorial has a great strategy to use a capacitor to ensure a consistent current.
More How To Mechatronics has a number of tutorials.
9v alkaline battery: 0.3 amp-hours, while delivering 100mA
AA battery 1.5v alkaline: 2.2 amp hours, while delivering 100mA
Rechargeable nickel-mrtal hydride battery: twice the endurance of alkaline
Lithium battery: three times the endurance of alkaline.
Source: Make: electronics by Charles Platt
The SparX Box is a great concept where the electrical components are in a box, easy to access and easy to remove. Love that they posted the CAD for it.
Lots of great information in this resource. New (and experienced) members of our robotics team need to read it.