Clinicians' patient care information needs are frequent and largely unmet. Online knowledge resources are available that can help clinicians meet these information needs. Yet, significant barriers limit the use of these resources within the clinical workflow. Infobuttons are clinical decision support tools that use the clinical context (e.g., institution, user, patient) within electronic health record (EHR) systems to anticipate clinicians' questions and provide automated links to relevant information in knowledge resources. This paper describes OpenInfobutton (www.openinfobutton.org): a standards-based, open source Web service that was designed to disseminate infobutton capabilities in multiple EHR systems and healthcare organizations. OpenInfobutton has been successfully integrated with 38 knowledge resources at 5 large healthcare organizations in the United States. We describe the OpenInfobutton architecture, knowledge resource integration, and experiences at five large healthcare organizations.