TinyScheme

What does /prague/ think of TinyScheme? It's a mostly complete R5RS interpreter, but without hygienic macros and syntax rules. It's painfully easy to embed into C or to extend with C, and is so small it can be shipped on anything, including micro-controllers with maybe a few hundred kilobytes of storage. It's also the most distributed version of Scheme, as it is the main scripting language for The GIMP, and was shipped to over 4 million computers with adware.

http://philosecurity.org/2009/01/12/interview-with-an-adware-author

Also, Chibi Scheme appears to have a dependency on POSIX, meaning that it cannot be used in Windows applications without using Cygwin, and it cannot be ported to more obscure systems (say, Amiga) that are not POSIX based.