How to open sitx files?

You can try 'The Unarchiver' http://unarchiver.c3.cx/ which claims to support decompressing .sitx

For installation, have a look at http://unarchiver.c3.cx/commandline

sudo apt-get install unar
unar archive.sitx

These are StuffIt files. It appears to be a proprietary format and unpopular enough that none of the open source archivers have ever bothered to reverse-engineer it only one application (see Sharath's answer) has ever battled it.

The Unarchiver does only claim partial support. If the archive uses the more advanced features like encryption or JPEG compression, by all means give it a shot, but you'll likely need to fall back to one of these options, none of them great:

• Use a real copy of Windows or OSX along with StuffIt Expander. This could be in a Virtual Machine.

• Battle Wine to the death. Stuffit Expander uses the .NET framework v4 which seems to mean "it's just not going to work quite yet". AppDb reviews seem to corroborate that.

• Find the person sending you such an obscure format and keep stroking their hair until they agree to send you a standard format like, you know, a Zip.