(De Bellis Antiquitatis)
DBA is a set of rules from Phil Barker of the Wargames Research Group which has proved to be very influential and enjoyable.  The concept is that every side possesses an equal amount of elelments - 12 to be precise, and that movement is dependent upon command and control.
These rules later lent themselves to larger scale rules, DBM and DBR for Ancients and Renaissance.
DBA remains a great introduction to the hobby of historical wargaming.  Set up costs are minimal as the most figures you will need for an army is 48.  This also means that not much painting is necessary before the armies take the field.  The osts are even less if a person builds their armies out of 1/72 plastic as I have done.

The Armies that I have are listed below. Many are a work in progress, but an element painted here and there adds up very quickly! There are links to the pages for the finished armies.

Later Carthaginian
  Polybian Roman

  Later Frankish

Using the Pike and Shot extension available on
Fanaticus I also have:

Below: A pike phalanx