The SCGI protocol is a replacement for the Common Gateway Interface (CGI) protocol. It is a standard for applications to interface with HTTP servers. It is similar to FastCGI but is designed to be easier to implement.
The is a short specification that defines the protocol. Also available is a package that implements the client and server sides of the protocol. The latest release is scgi 1.14 (sig). The development version of source can be downloaded using Git. Zed Shaw has win32 builds of mod_scgi on his site. The Lighttpd web server has built-in support for the SCGI protocol.
Some people appear to have trouble getting mod_scgi to work on MacOS. Martin Blom has provided a procedure that appears to work.