Hello!
.SIT is short for Stuffit wich is the application that you use to unstuff compressed .sit files and all other sorts of compressions like .zip etc
If you do not use a Mac however I doubt that you will find .sit files usefull because usually .sit files on Gnutella is Mac Applications and not for a PC.
If you are using a PC then you should look out for .zip files instead.
If you are on a Mac then you should already have Stuffit Expander installed since it is the industry standard for Mac's.
BUt you should go to stuffit's site and update to the latest Stuffit Lite package which is version 6.5.1 and you can download it from here:
http://www.stuffit.com