Open source projects are gradually incorporating usability methods into their development practices, but there are still many unmet needs. One particular need for nearly any open source project is data that describes its user base, including information indicating how the software is actually used in practice. This paper presents the concept of open instrumentation, or the augmentation of an open source application to openly collect and publicly disseminate rich application usage data. We demonstrate the concept of open instrumentation in ingimp, a version of the open source GNU Image Manipulation Program that has been modified to collect end-user usage data. ingimp automatically collects five types of data: The commands used, high-level user interface events, overall features of the user's documents, summaries of the user's general computing environment, and users' own descriptions of their planned tasks. In the spirit of open source software, all collected data are made available for anyone to download and analyze. This paper's primary contributions lie in presenting the overall design of ingimp, with a particular focus on how the design addresses two prominent issues in open instrumentation: privacy and motivating use.