autoCAD is the most common one, but a lot of practices are using vectorworks (my own included), microstation, some even use archicad.
using any of those packages as a base, sketchup is becoming a very popular tool for designers for basic renders. viz, studio max and other powerful 3d modelling and renderring tools have become the preserve of larger firms with dedicated inhouse 3d teams and firms whose sole purpose is architectural visualisation and animation.