ClearHealth is a comprehensive practice management and EMR system incorporating the key categories of functionality for scheduling, patient registration, electronic medical records and CPOE, electronic and paper billing, and SQL reporting. As an open source reference implementation of several interoperability protocols ClearHealth has support for working with data in HL7  and Continuity of Care Record (CCR) formats.
The ClearHealth system is fully compliant with HIPAA security provisions 
Written in the PHP language and capable of running on most server configurations, windows, Linux or Mac OS X, under Apache and MySQL (LAMP), ClearHealth is compliant with the expectations of most Open Source web-based systems. (source: http://en.wikipedia.org/wiki/ClearHealth)
Standards: CCR, DICOM, HIPAA, HL7, XML
ClearHealth has taken the powerful VistA EMR system which powers the Veterans Administration health network and modernized it. With added, seamless, scheduling and billing WebVista offers the only fully comprehensive VistA based system in a cost-effective, Web 2.0 package. Utilize all of the capabilities from a standard web browser.
The product comes into two version: Office and Avantage. The main difference is the amount of encounters the Office version can handle i.e. 10,000 while Advantage can handle >10,000 encounters. The following are the features of both as lifted from the official website itself:
The ClearHealth medical suite now offers a next generation set of capabilities to practices large and small. ClearHealth is a fully comprehensive system encompassing all of the following features.
Scheduling: Full Featured Scheduling & Calendars
Rooms & Resources
Full Featured Patient Registration
Health Status Alerts
IHI Self Management Goals
Real-time Lab Results
Electronic Lab Ordering
LOINC Code Support
Barcode Scanning and Workflows
ID Cards/Magstrip support
Patient Portal/History from Home
Data sharing/Patient Portal
Electronic Submission (X12)
PDF Specialty Forms
Custom Superbill/Electronic Superbill
User definable reports
Easy of deployment
ClearHealth is Open source EMR and clinic management software , focuses on the most needed big five features: Medical Billing, Medical Accounts Recievable, Scheduling, Access Control, and EMR . (http://www.goomedic.com/clearhealth.html);
An article entitled, "Introduction to ClearHealth" posted by David Uhlman on Thu 2nd Jun 2005 17:41 UTC URL http://www.osnews.com/story/10740/Introduction_to_ClearHealth/page1/
A study made on Vista and ClearHealth is available on this site:
http://www.springerlink.com/content/28m1t1k71142n21m/ (only abstract is available; the full text access will require payment)
A review by the California Health Foundation is found here: http://www.chcf.org/resources/download.aspx?id=%7B905B88C8-EC65-4CF0-886...