SIT are files that are archives created and maintained by Stuffit Expander. It is a format that resembles ZIP archives in structure. Multiple files and folders with subfolders can be packed into an archive. The SIT format was originally used in Apple systems and devices, but is also supported by Microsoft Windows platforms.

Stuffit Expander is a compressed archive management program that supports approximately 25 formats. The tool was developed by Smith Micro Software, Inc.