The PAPI project has defined and implemented a crossplatform interface to the hardware counters available on most modern microprocessors. The interface has gained widespread use and acceptance from hardware vendors, users, and tool developers. This paper reports on experiences with the community-based open-source effort to define the PAPI specification and implement it on a variety of platforms. Collaborations with tool developers who have incorporated support for PAPI are described. Issues related to interpretation and accuracy of hardware counter data and to the overheads of collecting this data are discussed. The paper concludes with implications for the design of the next version of PAPI.