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Alfredo Novoa's D
A Tutorial D on MS.Net with data independence as a main goal, running its own storage engine.
Alphora Dataphor
A fully-working C# and MS .Net-based implementation. Had to accept contamination by SQL NULLs due to using SQL DBMSs as storage engines.
Duro
A copylefted C library.
G-Exec2
A planned reimplementation of one of the first relational systems, the British G-Exec from the 1970s.
Opus
A copylefted C-like relational programming language with OO as a second design goal.
Rel
An in-progress copylefted flavour of Tutorial D implemented in Java.