University of Oregon
University of Oregon: Hipparchus's Life and Work
This site from the University of Oregon provides an overview of Hipparchus's life and his major accomplishments. A good starting point.
Wikimedia
Wikipedia: Hipparchus (Astronomer)
Encyclopedia article about the Greek astronomer Hipparchus that contains information about his life and work. Also includes a lengthy list of references and links to additional information near the bottom of the page.
Other
Fermat's Last Theorem Blog: Hipparchus of Rhodes
Brief biography of the Greek astronomer Hipparchus of Rhodes. Also contains one illustration and a couple of reference links.