On Windows, HFS+ can be read only with the aid of software such as MacDrive or MacOpener; it's possible to run MacOS 8.1 in emulation, but I know of no suitably old software that can open .dmg ...

A coworker want to burn some .dmg files for a client of his but he has has Windows (98, 2k or XP). Any way to do this? Does he need a buster of some sort like the .bin busters?

Several users have reported a problem where Firefox 1.5 has problems properly recognizing that files with the .dmg.gz extension (disk images that have been compressed using gzip).