| Sitx is a recent revision of the Stuffit format. It is not self-extracting by itself, although a free extractor program is available from aladdinsys.com.
.dmg is the Disk Image format, and mounts as a new volume when double-clicked, via Apple's Disk Utility or Disk Copy programs (depending on which version of OSX you're using). Hence, it's sorta self-extracting, but because it mounts as a drive icon, you have to copy the files to your hard drive as a seperate step. |