Dave's Famous Original implementation of the World's Most Wonderful Programming Language follows very closely (but substantially extends) the original SETL described in the textbook. It leaves out a few esoteric features like backtracking, the representation sublanguage, macros, and old-style separate compilation facilities.
Of the other descendants that I am aware of, SETL2 and ProSet merit the most attention.
In addition to the above, I am also aware of two scaled-down versions of SETL that run well on slow, small MSDOS systems: ISETL (supported by an excellent textbook) and SETLS.