Since I use almost exclusively hand tools..
Nibbler - takes small "nibbles" out of material up to about 1/16 thick. Some materials and angles will be harder than others but if you aren't able to use power tools it will get the job done. Also good for squaring round holes before fine tuning with files.
pipe reamer - deburs both the inside and outside of the a tube. Useful when using a pipe cutter to cut your tubing, can also be used for a slight chamfer. (pro tip, run it backwards for softer materials)
Step bits - Not really obscure, but I think a lot of new builders aren't aware of them
1/2" PVC pipe - enough to stick inside a 1" thin blade tube. This will make it possible to quickly and easily cut down your blades with a pipe cutter.
3D printed jigs - use your 3d printer to make useful tools as well as parts!