http://en.wikipedia.org/wiki/Archive_Utility
Took a little research, but wikipedia came through. I chose to paste link rather than just plagiarize. I really didn't want to install a stand alone stuffit when I knew I'd opened .sit files before. Just choose open with on a .sit and navigate to /System/Library/CoreServices/Archive Utility.app
Make that the default and your extension is restored to a seamless built in function of OS X.