For Unix, ispell is probably the program of choice. It is
available in support/ispell; beware of any version with a number
4.x - such versions represent a divergent version of the source
which lacks many useful facilities of the 3.x series.
For MS-DOS, there are several programs. amspell can be called from within an editor (available as support/amspell). jspell is an extended version of ispell (available as support/jspell).
For the Macintosh, Excalibur is the program of choice. It will run in native mode on both sorts of Macintosh, and is available as systems/mac/support/excalibur/Excalibur-2.5-sit.hqx (there are other dictionaries in the same directory).
For VMS, a spell checker can be found in support/vmspell