The Solitary Wargamer offers a variant of DBA for the English Civil War.
DBA
DBA 3 Reference Sheet
The Society of Ancients offers a free DBA 3 reference sheet.
DBX Colonial Rules
Pushing Tin offers a set of colonial rules in the DBX style.
Napoleonic DBA Rules
Steve Burt has a set of free wargames rules for playing Napoleonics using DBA mechanics.
De Bellus Middle Earth Rules
De Bellus Middle Earth is DBA variant for playing wargames set in JRR Tolkien’s Middle Earth.