CGL - An ANSI C library for CGI development with a simple and clean interface, as well as default and configurable data limits. [Open source, BSD license] - http://www.harding.motd.ca/cgl/
cgi-util - C library for creating CGI programs. - http://www.newbreedsoftware.com/cgi-util/
CGIC - Thomas Boutell's ANSI C library for CGI Programming - http://www.boutell.com/cgic/