In this paper we present OpenSoT, an open-source, recently developed software library, that can be used to solve robotics related control problems in a flexible and easy way. OpenSoT includes high-level interfaces to state-of-the-art algorithms for kinematic/dynamic modelling, quadratic programming optimization, cost functions and constraints specification. OpenSoT is implemented in C++ and permits rapid prototyping of controllers for fixed or floating base, highly redundant robots such as (but not limited to) manipulators and humanoids. We discuss the use of OpenSoT from the perspective of the developer and the user, leaving out details on the implementation of the tool. We demonstrate how the software can be used with two examples: control of a redundant humanoid robot through simple inverse kinematics schemes and contact forces optimization.