Sketch-based tools provide a more human centered design environment than traditional widget-based computer design software. A number of sketch tools exist that support specific design tasks: however wider exploration of computer supported sketching is being hampered by the effort required to build the sketching software. Here we present a sketch tool framework, its implementation and evaluation. The implementation, InkKit, provides context free design spaces and a powerful, trainable and extensible modeless writing/drawing recognition engine. It reduces the development effort for a specific diagram type from thousands of lines of code to a few hundred. We evaluated our toolkit by asking fourth year computer science students to use InkKit to develop a diagram specific recognizer.