Ok, many people have probably heard of the Land Value Tax or Georgist tax.
http://en.wikipedia.org/wiki/Land_value_tax
Ars-Regendi definitely needs it, either as a reform or task option. The LVT would be excluded by Free land reform, Anarcho-capitalism, and Planned Economy. As for effects, I'll leave that for the devs to decide; obviously it's got to impact the GINI toward the left side.