Sparkle URL: http://polloux.fciencias.unam.mx/~quartic/gimp/sparkl/sparkl.tar.gz
John hasn't got the source online yet, but when he does I'll put its address here. His Home Page is at http://jumpjibe.stanford.edu:8080/beale/.
The Sparkle source is not a single file, as most other plug-ins are. It is a set of files, thus the need for a compressed tar file. I had to get rid of the Humor section of my personal pages in order to make room for the various source I've written. Bummer.
Building the source should be fairly straight forward. Just make sure the LIBS= line points to the location of libgimp.a. Also, a copy of gimp.h is included in the source. You should really delete this and make gimp.h a symbollic link to your copy of gimp.h (wherever you're keeping that) so that the latest version of gimp.h is used.