APN Hoedown is a Markdown processing library written in portable C.
A critcl generated wrapper for hoedown is around is available from my fork of the project on github. To build, checkout the vc6-compatibility branch (the wrapper has not been merged into the master branch as yet though it will build on any critcl target platform). Then cd to the tcl subdir and type
tclsh build.tcl -target WHATEVER