pwntools. A python package used for exploit development. Contains a bunch of super useful modules, from creating elf files to helping find offsets of functions in files.
https://github.com/Gallopsled/pwntools
https://github.com/Gallopsled/pwntools-tutorial

ROPgadget. A tool for finding ROP gadgets in a binary. Makes writing ROP chains doable rather than incredibly hard.
https://github.com/JonathanSalwan/ROPgadget


one_gadget. A tool that makes ROP simpler by listing all gadgets that simply call system(/bin/sh). Don't become overly dependent on this, though it can be nice to avoid wasting time writing ROP chains.
https://github.com/david942j/one_gadget