A great site dedicated to Outlining software (especially older versions). Covers both Mac and PC (DOS & Windows) with downloads. See Outliners.Com: Welcome to Outliners.Com!
Sure you can do outlining in Word but I’m a big fan of many small, different, efficient and dedicated pieces of software working together to get the job done (as in the “Body of Christ”?). Must be the UNIX background coming through.