Author of safe Tcl, a system for secure execution of scripts, later folded into the Tcl core as part of the work on interpreter hierarchies (slave interpreters, see interp).
Marshall's done much else besides. He wrote early SNMP- and MIME-handling software in Tcl. The MIME-handling code was added to tcllib, see the module of the same name. This code also provides the smtp package.
See also his BOOK How to Manage Your Network Using SNMP.
See also BEEP.