The Anderson Stuart Building
The Anderson Stuart Building (formerly known as the "Old Medical School") is a superb example of neo-gothic architecture located next to the main quadrangle.
The building, carved out of beautiful golden Sydney sandstone - exhibits many gargoyles, superb stained glass windows and carvings. It was designed by James Barnet, with additions by Walter Liberty Vernon and then Leslie Wilkinson. It is on the Heritage list and has recently undergone substantial renovations.
You can have a virtual tour of what the Anderson Stuart Building looked like in the past here (external page).
For a Birds eye view of the building, have a look at Google Maps.
You can see the buildings location in relation to the University of Sydney grounds on the campus map.