The Hubertuswarte tower stands at the highest point of Lainzer Tiergarten, the top of the Kaltbründlberg (508 meters above sea level). It is an 18 meter tall observation tower that was built in 1927, and from the top of the tower one gets a 360 degree view of the Tiergarten, with Vienna and neighboring towns seen in the distance, and on a good day it is said[a] that one can even see the Ötscher (mountain).
Update 2008-04-24: I've been here several times since this entry was written. This search for Hubertuswarte should show them all. Physically getting to the Hubertuswarte is easy: head for the Rohrhaus and then take the trail south; but if you intend to stay for the sunset be ready for a rapid march to the nearest exit, as you risk missing the closing time.