Kolo33's AT-AT model has some nice design detail but is hard to print, and the pose is somewhat arbitrary. This is an improved version.

I fixed problems related to too-thin geometry, strange internal faces, areas that didn't join properly and so on. I also divided the model into parts based on the natural joints and added allowances so they can rotate. This allows the model to be posed, although the final pose will need to be glued to be able to hold its own weight.

Completed model stands 30 cm high. Based on: http://www.thingiverse.com/thing:215574

• head, gun_L, gun_R: glue guns into holes

• body_front, body_mid, body_back: glue together, attach head
  some alternate orientations are provided

• chassis_front, chassis_rear: glue to base of body

• chassis_plug x4, leg_upper x4: glue plug into chassis so leg can rotate
  these are mirror-symmetric

• leg_mid x4: slip on upper leg joint

• for the bottom leg you have two options:
  -- legA_L, legA_R: left and right versions of the bottom leg segment and foot, with the foot centered
  -- legB_main, legB_foot, legB_insert, legB_strut, legB_strut_peg: glue insert into main allowing foot to rotate. Once foot is posed glue strut and peg into place. A short piece of filament instead of the strut peg works great

• pose entire assembly, glue all joints into place