s9fes is another small Scheme that runs on Unixes, OS X, Plan9 & Windows. The code is in public domain and fully described in a very nice book "Scheme 9 From Empty Space: a guide to implementing Scheme in C". It weighs in at roughly 25% more lines than tinyscheme for *.{h,c,scm}.

http://www.t3x.org/s9fes/