VAERS is the Vaccine Adverse Event Reporting System put in place in 1990. It is a voluntary reporting system that has been estimated to account for only 1% (read more about underreporting in VAERS) of vaccine injuries. OpenVAERS is built from the HHS data available for download at vaers.hhs.gov.
The OpenVAERS Project allows browsing and searching of the reports without the need to compose an advanced search (more advanced searches can be done at medalerts.org or vaers.hhs.gov).