Although there is already a package in the tcllib library, huddle, that can generate JSON, the big advantage in using yajl-tcl is performance. Also, yajl-tcl can parse JSON as well as generating it.
In a benchmark generating JSON representing 5,000 airports with code, name, latitude and longitude, yajl-tcl was 157 times faster than huddle.
A webpage on yajl-tcl with example use can be found at http://www.yqcomputer.com/
The github project is at https://github.com/lehenbauer/yajl-tcl
A source tarball can be downloaded from https://github.com/downloads/lehenbauer/yajl-tcl/yajl-tcl-1.2.tar.gz or you can clone the git repository using "git clone git://github.com/lehenbauer/yajl-tcl.git"