Handling Apple’s .dmg files in Windows
If you just want to look into (or extract) contents of .dmg file, a good, old and free 7-Zip will take care of this. This should also work for Apple .sparseimage files. But, beware that (just like in case of .iso files for Windows) these formats can contain bootable information and or whole disk images (Apple .hps files), rather useless under Windows. So, for example, if someone tell you that he has Windows XP image ready to be used in VirtualPC for Macintosh, keep in mind that you rather won’t be able to use in in Windows version of VirtualPC. Or you’ll have a lot, lot of work with extraction, conversion, tune-up etc.
For mounting .dmg files you can consider piece of shareware called MacDrive. Opinions about mounting Macintosh images under Windows are different. Some people say that it is impossible, because these images uses HPS+ file system (unreadable under Windows). Others claim that software like MacDrive or even PowerISO should be able to do this. I was only interested in getting into contents of .dmg image, not mounting it, so I didn’t check this issue. You’ll have to discover this by yourself.
If you want to make more complicated operations on such files, shareware TransMac or PowerISO should be a good choice. But, bare in mind, that evaluation versions of these programs are limited to certain image size or certain group of operations. So, for large-scale system images these programs will still be rather useless, until you purchase full version of any of them.
If you want to convert .dmg file to .iso image (bare in mind what I wrote about bootable sequences or whole disk images) then you may consider:
First three are shareware. Last one is free, but unfortunately seems to be Apple application and there were reports on some forums that it fails to handle certain .dmg files correctly. Again bare in mind that evaluation versions of these programs are limited to certain image size or certain operation.
For burning .dmg files to CD or DVD again consider MacDrive or try to convert this image to .iso and then burn it with any software that allows burning .iso images. There are also rumors (unconfirmed by me) that most burning software can handle burning .dmg files painless, treating them just like .iso files (without conversion at all or with on-the-fly conversion during burning). You only have to rename the file or change its extension sometimes.