Eric did mention "Myth2_demo.img.sit" in the other post about downloads.
I had the same puzzlement with .sit files when our iMac came & I needed a Samsung printer driver.
Eric - .sit files are a compressed format - like .zip files, except you need a program called Stuffit Expander to unzip them. ( are there other ways?)
From my limited knowledge - it seems that .sit was the standard Mac compressed format for years & years & the program came with the OS - now it's not, & all the software suppliers are s'posed to use .dmg - but you'll still find some downloads as .sit files.
http://www.versiontracker.com/dyn/moreinfo/macosx/182