mod_rivet brings together the full power of the Tcl programming language
and the Apache HTTP server. You can use Tcl to manage Apache, respond to
requests for web pages and much more.

mod_rivet gives you a persistent Tcl interpreter embedded in your web
server. This lets you avoid the overhead of starting an external interpreter
and avoids the penalty of Tcl start-up time, giving you super-fast
dynamic content.

As you'd expect from the Tcl community, there are hundreds of modules
written for mod_rivet, everything from persistent database connections, to
templating sytems, to complete XML content delivery systems. Web sites like
FlightAware use mod_rivet.