All three of these handy utilities can be found on the Rainbow Home Page.
A simple drag-and-drop type application for converting DCM files (created by Disk Communicator--an Atari disk compression program) to ATR files (a common Atari disk image format used with emulators, etc.).
Atari Virtual Disk Editor, written by Richard A. Carvel. Can open Atari disk images in several common formats (ATR, XFD, ATD). Files within a disk image can be moved, deleted, or added using simple cut/copy/paste commands. Also includes a sector editor and can do basic text file conversion between Atari and Mac files. (Freeware)
A general purpose hex file editor which can be handy for tweaking files transferred from the Atari (ROMs, etc.). (Freeware)