Hyper Beam and switch PP underflow glitches + PP reset trick (Generation I)
For more information, please see the following links in the description (except for the third demonstration, which I've explained in this description):
First demonstration (Hyper Beam automatic selection and PP underflow) -: http://glitchcity.info/wiki/index.php/Hyper_Beam_automatic_selection_glitch
Second demonstration (Switch PP underflow glitch) :-
http://glitchcity.info/wiki/index.php/Switch_PP_underflow_glitch
In short, these glitches let you use a move with 0 PP and get 63 PP on the move with either all PP Ups applied or one PP Up removed.
Third demonstration :-
This is yet to be written on the wiki. The PP underflow glitches not only allow you to get 63 PP with full PP Ups applied/one removed, but allow you to remove the effects of all three PP Ups.
If you have 1 PP with three PP Ups used then the PP will look like this in hexadecimal: C1. Do this glitch, and it will become BF (63 with two PP Ups used).
From there, if you use a PP Up all PP Up boosts will disappear. This is because using a PP Up not only adds 40h (making this FF), but adds some remaining PP like an Ether. This makes the PP overflow and become something like 01 (1 PP with 0 PP Ups used). The game doesn't stop this with the "PP is maxed out" message because the PP was less than C0.