You might want to check out Racket. It's not immediately obvious that it's a Scheme, but it has a much more vast ecosystem of libraries than any other Scheme I know of.
Racket qua Racket (and PLT Scheme before the rename) didn't conform to any of the existing Scheme specs (it was something like R5RS minus some things it didn’t like, plus some things from R6RS, plus a whole bunch of its own stuff.)
After looking at the Racket website, from the POV of someone who wants to use or learn Scheme, that doesn't sound like a useful distinction. They have R5RS and R6RS implementations that look to have reasonable conformance with the corresponding standards. I don't see how the fact that the language called "Racket," which is a lispy language that implements (among other things) R5RS, R6RS, and mzscheme even matters. It sounds an awful lot like like saying "foo-scheme isn't a scheme because it's implemented partly in C."
