Cinema 4D has a pretty decent poly reduction function built right in, as do many of the better 3D apps such as Lightwave, etc. That being said, most people who reduce models "professionally" do it by hand. Building LOD (level of detail) versions of game models, for instance. While some software will do a pretty good job reducing fairly regularly shaped objects, you will inevitably need to go in and do some tweaking by hand to get the best results. Best of luck