How does lean compare to other improvement processes such as Six Sigma or Theory of Constraints?

While there are many specific differences among the different schools of thought, Jim Womack cautions against getting lost in the competing schools. For veterans of each practice often get lost in finely detailed arguments over technical or even philosophical differences. In an e-letter outlining the key differences, he nonetheless grounds the discussion by saying, “At the end of the day we are all trying to achieve the same thing: The perfect value stream.” His letter gives a nice overview of how to view each approach.
Quality Progress magazine published an artcle How To Compare Six Sigma, Lean and the Theory of Constraints which offers a very good overview that can help you choose the best framework for your organization.