Swank-gambit is a project mainly written in Scheme, it's free.
Gambit swank backend for SLIME
This is an initial version of a Gambit Scheme swank backend for SLIME.
Make sure to setup SLIME in Emacs:
(require 'slime) (slime-setup '(slime-repl))
Run swank-gambit.scm from the command line, and then type M-x slime-connect ENT ENT ENT to get a SLIME prompt.