Prior to the founding of Monte Alban, the large hill-top capital, the main center of the Zapotec civilization was in the Etla valley, west of where Oaxaca city is now. The site has several pyramid-platforms, a main square and a ball court. Excavations were carried out in the 1960s, but the site was never re-constructed to the extent of other sites in the Oaxaca valley, so a lot of it is difficult to make out, but it makes for an interesting stop on a day tour to Etla.
You shouldn't miss the community museum, which has a few striking pieces from the site, including the "diablo enchilado," a brazier that is painted red, and the carved slab known as Monument 3, which is similar in style to the depictions known as "Los Danzantes" of Monte Alban. The museum is often locked, but if you ask someone in the community they will find the custodian who will come and open it for you.