Rekall does not provide the DBMS, just the front-end required to maintain and report on it. It currently supports xBase, MySQL, and PostgreSQL "out of the box." Commercial plug-ins are available for Rekall to add DB2, ODBC, and (soon) Oracle access as well.
As of the beta 2.2 version of Rekall, which is to be released today, the product will be available under the GPL as well as under terms of a proprietary license. Support and source code for the free software version will be available at a new community portal sponsored by TheKompany.com.