ROM of the Month
Paper Mario does not have very many hacking tools yet with only two being fully developed.
A tool made by skelux. Originally titled Origami 64 and later renamed Kirigami 64 is a tool to edit battle formations text and more.
The tool can be dowloaded here.
A very interesting tool. It's quite similar to Mario Mache, but has some advanced features like having the power to edit store items.
A very advanced tool named Star Rod, still actively developed. It can edit pretty much every aspect of the game, including maps, battles and more.