Others have reported a problem with Stuffit Expander version 10 causing this behavior. Evidently it installs a buggy contextual menu item. See:
http://www.macupdate.com/reviews.php?id=9248
On my system, I removed the file "StuffitCM.plugin" from the folder "/Library/Contextual Menu Items" and rebooted. Now my contextual menus work again. I tried just the Stuffit Expander app on an archive and it seemed to work fine.