There are zillions of Scheme interpreters. It seems that everybody, at some point in their lives, write or at least try to write a Scheme implementation. -- Well, I haven't written one yet, but I have ported one to Minix. Lost the source when my laptop was stolen (aeons ago), and then lost interest. (Why Minix? The beast had only 2 MB of RAM and couldn't run anything else :) )

schemers.org is a good resource for schemers. The FAQ lists some of the implementation. For a quick start try this Java applet straight from your browser. Did you know that you can have a complete Scheme implementation in your pocket?