CGI

The CGI Resource Index: Programs and Scripts: C and C++
Links to useful web scripts coded in C.
http://cgi.resourceindex.com/Programs_and_Scripts/C_and_C++/

Form Mail
Widely-used forms-to-mail program for Unix written in C.
http://web.mit.edu/wwwdev/cgiemail/

Global Computing System (GCS)
A client-server system where the client writes his C program in the textarea form field. [Online demo]
http://home.att.ne.jp/omega/kikuchi/gcs.html