In this paper, we present CamiTK, a specific modular framework that helps researchers and clinicians to collaborate in order to prototype Computer Assisted Medical Intervention (CAMI) applications by using the best knowledge and know-how during all the required steps. CamiTK is an open-source, cross-platform generic tool, written in C++, which can handle medical images, surgical navigations and biomechanical simulations. This paper first gives an overview of CamiTK core architecture and how it can be extended to fit particular scientific needs. The MML extension is then presented: it is an environment for comparing and evaluating soft-tissue simulation models and algorithms. Specifically designed as a soft-tissue simulation benchmark and a reference database for validation, it can compare models and algorithms built from different modeling techniques or biomechanical software. This article demonstrates the use of CamiTK on a textbook but complete example, where the medical image and MML extensions are collaborating in order to process and analyze MR brain images, reconstruct a patient-specific mesh of the brain, and simulate a basic brain-shift with different biomechanical models from ANSYS, SOFA and ArtiSynth.