A .sit expander?

Why not use the carbonized version of StuffIt Expander that's included with Mac OS X Beta? Go to your Applications folder, open the Utilities folder, and open the Aladdin folder. Enjoy.

Don't use StuffIt Expander to expand .tar.gz or .tgz files though. It doesn't like those for some reason. Either use the BSD utilities in the Terminal or get OpenUp. (OpenUp also handles many other archive formats.)

What version of OpenUp will work with OS X PB? Their site has an OS X Server version and an OS X DP4 version.