Dataphor: A free application development platform streamlining the design, development and maintenance of software applications

FirstSQL: Java in-memory SQL92 DBMS with sane NULLs processing. Purports to actually strive to follow Codd's rules

LEAP: An incomplete relational algebra DBMS. [Free software, GNU GPL

MighTyD: A prototype for demo purposes, the first implementation of the temporal database model devised by Lorentzos, Darwen and Date. Uses Tutorial D and PostgreSQL, being compliant with The Third Manifesto