Test lights are excellent tools and a must for any trouble shooting.
Multimeters are another invaluable tool when working on any electrical circuits.
Power probe is a very good tool but more of a luxury but when you have one they can be very useful indeed.
Noid light these are useful for checking the operation of injector wiring and signal if the injector is getting a pulse to fire from the cdi or not and if the wiring is intact.
Jumper wires and pins are needed to help access wires and terminals with minimal fuss and damage to the loom.
Battery testers are useful but you could use multimeter and get a similar result without the extra costs involved.
Oscilloscopes are very useful for picking up glitches and injector faults and fault on sensors like pick up coils etc but they are veery expensive so not needed for DIY.
Soldering tools are excellent for making permanent reliable fixes in the wiring when repairing damage wires or replacing components.