Capacitive sensing allows the creation of unobtrusive user interfaces that are based on measuring the proximity to objects or recognizing their dielectric properties. Combining the data of many sensors, applications such as in-the-air gesture recognition, location tracking or fluid-level sensing can be realized. We present OpenCapSense, a highly flexible open-source toolkit that enables researchers to implement new types of pervasive user interfaces with low effort. The toolkit offers a high temporal resolution with sensor update rates up to 1 kHz. The typical spatial resolution varies between one millimeter at close object proximity and around one centimeter at distances of 35 cm or above.