Outlook Personal Store (.pst) to Mail Box (.mbox) converter with nice simple drag&drop interface
On OS X you could somehow install some program (readpst) via homebrew or macports that does exactly the same thing... but not every user feels comfortable using a terminal.
Plus I didn't find any alternatives for converting Outlook .pst files to .mbox files on OS X
Disclamer: it's an old program of mine (from 2011) or something....