I'd recommend Gambit-C scheme:
- It's R5RS-conformant.
- It has both an interpreter and a compiler. You can also compile to ANSI C.
- It's open source.
- It's portable. (It runs on Linux, Windows, Mac OS X and even iOS.)
- It has simple foreign function interfaces (FFI).
A cursory examination reveals that Chicken seems unsatisfactory, while Bigloo may be a serious contender. But I cannot comment too much about them.
