From the Website at http://code.google.com/p/chibi-scheme/
Chibi-Scheme is a very small library intended for use as an extension and scripting language in C programs. In addition to support for lightweight VM-based threads, each VM itself runs in an isolated heap allowing multiple VMs to run simultaneously in different OS threads. The default language is R5RS Scheme, including first-class continuations and low-and-high-level hygienic macros.
The system is designed in optional layers, beginning with a VM based on a small set of opcodes, a set of primitives implemented in C, a default language, a module system implementation, and a set of standard modules. You can choose whichever layer suits your needs best and customize the rest. Adding your own primitives or wrappers around existing C libraries is easy with the C FFI.
Most features are optional at compile time. A full-featured build produces a library just over 100kb, whereas a minimal build can be under 80kb.
Chibi-Scheme is known to work on 32 and 64-bit Linux and OS X, Plan9, Windows (using cygwin) and iOS.