GNU Health - GNU Health is a Free/Libre project for health practitioners, health institutions and governments.
OpenMRS - OpenMRS is a community-developed, open source, enterprise electronic medical record system platform.
drchrono - An EHR built for your specialty:
Kareo - Kareo - Go Practice | Medical Office Software for Small Practices
Solismed - A free clinic management system designed for use by independent practices and free clinics. It's designed to automate your business operations and help you get things done more quickly.
Castor EDC - Castor offers you a user-friendly and fully featured application for electronic data collection.