Rob, the host of Atari’s Player/Missile Podcast has been working hard on Omnivore, a cross-platform utility for modern hardware (running Linux, OSX, and Windows) to work with executables or disk images of Atari 8-bit machines.
It is actually a very versatile tool since it is, at the same time, a hex editor, a disassembler (with mini-assembler) and a map editor.
Developed in Python, the tool source code is available on Github. The author states that although very complete, Ominivore is still in beta so bugs might exist. He asks for feedback from whoever uses the tool.
On the Player/Missile podcast website, there is a blog where you can follow new developments of the app.