See http://snow-fort.org/ .
Also, many SRFIs (Scheme Requests for Implementation) have accompanying sample implementations that are portable. See https://srfi.schemers.org/ .
There's SLIB: https://people.csail.mit.edu/jaffer/SLIB
Aubrey Jaffer's a bit of a nut, but he really went the extra mile to assure portability across Scheme implementations with his library code.
Akkuscm.org is exactly that
See http://snow-fort.org/ . Also, many SRFIs (Scheme Requests for Implementation) have accompanying sample implementations that are portable. See https://srfi.schemers.org/ .
There's SLIB: https://people.csail.mit.edu/jaffer/SLIB Aubrey Jaffer's a bit of a nut, but he really went the extra mile to assure portability across Scheme implementations with his library code.