Harbour is a free software compiler for the xBase superset language Clipper.
Harbour team very hardly worked ( and they’re still working ) and had included various extensions of Clipper language; such as class creation, support for date constants, FOR EACH and SWITCH structures, many extensions to RDD and language features and much more.
The main advantage Harbour has over other Clipper compilers is that it is free software. Harbour also attempts to remove some of the limits imposed by the base implementation but the extent of this will depend on your chosen platform.
Most importantly, try Harbour for yourself, decide for yourself why it has advantages for you.
Harbour has a fork : xHarbour.