While Realtalk OS is not open source there are open source projects that allow you to build mini subsets of Dynamicland. La Tabla[1], Paper Programs[2] and nl-datalog[3] are either prototypes of systems the eventually came together to build Dynamicland or in the case of Paper Programs an implementation loosely inspired by Dynamicland.

[1] http://tablaviva.org [2] https://paperprograms.org [3] https://github.com/harc/nl-datalog