From Poser and Daz Free Resources
StuffIt Expander
Download Links
Main Site:StuffIt Expander Mac
StuffIt Expander Windows

StuffIt is a file-compression utility, currently sold by Smith Micro. The format was popular on pre-OSX Macintosh systems.

There are free "expander" programs available for opening .sit files:

Those pages also have links to the full (paid) versions of the software, which allow for creation of .sit files.