broadly i could say, clip is a very very complete clipper compiler for
unix-like enviroments.
Harbour is a more traditional implementation but multiplatform, and
xHarbour is like Harbour but not restricted to clipper original
implementations.
two years ago, after using harbour and xharbour i decided to 'marry'
clip, and i'm very happy with my decision. their (clip authors) english
is worse than mine, and development is closed, but as compiler and
extensions imho clip is the best of the three.
http://www.itk.ru/english/index.shtml
http://www.xharbour.org/
http://www.harbour-project.org/