Formerly known as Sator, this engine supports JSP 1.1 atop Servlet API 2.0 or later. You may wonder how that can work since JSP 1.0 and 1.1 depend on Servlet API 2.1. The answer is that SJSP on top of Servlet API 2.0 doesn't support JSP forward and include functionality. They also have a version that supports JSP 0.92 legacy code. Free for development, contact for commercial use.
Commercial users get source.
Includes a program that converts .jsp files to .java servlets directly.