R is the free version of the S language. S-PLUS is a commercial version. Both are targeted at statisticians per se. Their strengths are in exploratory data analysis (in my opinion).
SAS has many statistical featues, and is phenomenally well-documented and supported. One of its great strengths is the robustness of its data model -- very well suited to large sizes, repetitive inputs, industrial-strength data processing with a statistics slant. Well over 200 SAS books,for example. I think of SAS and R as being like airliners and helicopters -- airlines get the job done, and well, as long as it's well-defined and nearly the same job all the time. Helicopters can go anywhere, do anything, but a moment's inattention leads to a crash. -- http://mail.python.org/mailman/listinfo/python-list
