Rivet is a break from the past, in that we, the authors, have attempted to take what we like best about our past efforts, and leave out or change things we no longer care for. Backwards compatibility was not a primary goal when creating Rivet, but we do provide this information which may be of use to those wishing to upgrade from mod_dtcl or NWS installations.
Rivet was originally based on the dtcl code, but it has changed (improved!) quite a bit. The concepts remain the same, but many of the commands have changed.
NWS was a server-side scripting environment based on the Apache HTTP server and Safe-Tcl and it's not maintained anymore.
The biggest difference between Rivet and Neowebscript is that Neowebscript was designed for shared hosting while Rivet is more oriented toward a site that owns all of its content. In the words of Karl Lehenbauer, NWS author and Rivet Team member:
Rivet, however, is considerably more efficient, powerful, and evolved, though it still can support multiple independent sites on a single machine through its “separate virtual interpreters” mechanism.