I've made a list of Scheme Implementations at this incredibly ugly URL: http://spreadsheets.google.com/ccc?key=0Aozj6rF-PmISdHdSU1duajFoLWpfRjNJSFJYeHdybWc.
This list is not meant to be exhaustive, but I'd like it to include all the Scheme implementations that are likely to affect, or to be affected by, the next Scheme standard.
For the moment, I just want to make a list of active implementations. I don't want to compare features, or list conformance. I'll add these later if they seem relevant.
Please send me email if I have omitted things, got things wrong, or included abandoned implementations. Thank you for your time.