It's a C. Lawson and R. Hanson Fortran 77 code published in "Solving Least Squares Problems" translated to C code and binded to Ruby.
$ gem install nnls
a = [0.5, 0.3, 0.2,
0.2, 0.7, 0.8]
b = [0.1, 0.1, 0.7]
result = NNLS.nnls(a, b, 2, 3)
=> [[0.05263157894736844, 0.0, 0.10526315789473684], 0.0]
This gem is distributed under MIT license.