DMG file with .dmg file extension is a Apple compressed archive format which is treated like a real volume or disk image on Mac OS X. DMG file is normally created with Disk Copy of Macintosh system, and with the same utility, it can be burnt to CD/DVD or mounted as normal volume. When used for application distribution, the Mac supports direct execution of setup installer within the DMG.
For people who downloaded a .dmg file onto a Windows PC computer, chance is that the .dmg file cannot be opened or viewed in Windows directly. If you have no a Macintosh computer, there are several software utilities which able to read, open, convert or burn the .dmg file directly on Windows system.
dmg2img is a tool which can convert the .dmg archives into standard (HFS+ or HFS Plus) image disk files in .img file format. It supports handling of zlib, bzip2, UDCO compressed DMG images. Though, image produced dmg2img can only be mounted under Mac OS X or Linux with hfsplus support, there is a legacy dmg2iso which can converts zlib compressed DMG images to ISO images directly, though it no longer be supported.
DMGExtractor is a lightweight Java application that able to extract and convert the contents of a Mac OS X Disk Image file (extension .dmg) to raw binary format (such as an ISO file). It can handle most disk images of type CUDIFDiskImage, with the exception of UDCO images. In addition, DMGExtractor can handle AES-128 encrypted and password-protected DMG disk images.
HFSExplorer is an application that can read Mac-formatted hard disks and disk images. It can read the file systems HFS (Mac OS Standard), HFS+ (Mac OS Extended) and HFSX (Mac OS Extended with case sensitive file names), including most .dmg disk images created on a Mac, including zlib / bzip2 compressed images and AES-128 encrypted images.
PeaZip is an open source file and archive manager which supports unpacking and extracting of DMG disk image file format.
7-Zip is a file archiver with a high compression ratio, which supports unpacking, decompression and extraction of DMG disk image format.
IsoBuster is actually a CD/DVD and BD/HD DVD data recovery software that can interpret, open and extract various CD/DVD/Blu-ray disk image files, including DMG.
There are other commercial software which can open, extract, view .DMG image file or convert the DMG to ISO and even directly burn the DMG to CD/DVD/Blu-ray, such as Acute Systems TransMac, UltraISO, PowerISO, MagicISO and MacDrive, where the later can also mount simple DMG files as drives under Windows. Most commercial applications offer free trial period which the software can be downloaded, installed and used without limitation before having to pay for a license.