ZDevtools consists of an assembler and disassembler for the Z-machine. They were originally developed to support the implementation of the Bocfel Z-machine interpreter. These tools are not particularly useful in the general case, and people will generally fare better with programs like Inform, ZILF, ZAPF, and txd. One major feature that these do include, beyond what is available elsewhere, is the visual disassembly mode.
ZDevtools can be downloaded from the downloads page.
Issues can be reported here.