Judging from the number of games I’ve seen at conventions, De Bellis Multidudinus (DBM) is probably the most popular ancients game out there. Unfortunately, it’s written in an obscure dialect of English known as Barkereese. Linguists contend that the form originally developed as a form of cryptography, allowing the Angles to the Saxons in the dark. Unfortunately, it also keeps gamers from knowing exactly what the author intended. As a result, many scholars have dedicated their websites to explaining it all. This site is full of diagrams explaining DBM’s concepts to those not fluent in Barkereese.
Based on DBA, De Bellus Mortuus is a set of free wargames rules for a fun zombie game. Author Chris Brantley writes:
This “just for fun” DBA adaptation is inspired by all those great zombie movies, ranging from the classic Night of the Living Dead to the comedic Sean of the Dead, as well as by the gamers who gather to hack and blast their way through legions of plastic zombies during local conventions. In this game, Players take sides, commanding either the forces of “The Living” or “The Undead.”
Chris Brantley has collected this list of rules variants for the De Bellis Antiquitatis (DBA) ancients miniature wargame.
David Kuijt has instructions on building terrain squares for DBA miniatures games.
De Bellis Vietnam is a set of DBA-based quick and dirty rules for wargaming Vietnam.