R6rs scheme's (or even better Racket's) macro system. Recently similar systems have been making their way into other languages, but the AST-like syntax of lisps

R6rs scheme's (or even better Racket's) macro system. Recently similar systems have been making their way into other languages, but the AST-like syntax of lisps make it a fair bit easier to use. Other than that? Delimited continuations. Guile-fibers, a parallel concurrent ML (which I would describe as a generalisation of go's concurrency model) for guile scheme is implemented using them as a scheme-only library. No low level voodoo. Just straight forward scheme.