Pika Scheme is a new implementation created by Tom Lord of Arch? fame (Arch? is a version control system). Currently, it is a work-in-progress -- not yet a useful implementation.
See Tom's homepage. But most of the pika links are broken/unimplemented placeholders.
There are also prereleases, a mailing list, and a project on Savannah?. The code can be found at these Arch archive coordinates:
Tom's homepage says "I am cautiously optimistic that it will be a complete implementation this year, hopefully by the end of the summer". It is not clear whether this was written in 2004 or 2005.