In addition to Emacs:

 - There's the SchemeWay plugin (http://sourceforge.net/projects/schemeway/) for Eclipse (http://www.eclipse.org/), using which you can edit Scheme code. It highlights the other parenthesis, does automatic code formatting using the Tab key, and has syntax highlighting.

 - A real basic alternative for minor hacking could be Vim (www.vim.org) - it highlights the other parenthesis, and has syntax highlighting.

 - Jazz Scheme has an editor

Mikael