As far as weightlifting goes (cardio being a different story), it's one day of heavy lifting followed by one day of rest, no exceptions. Many argue that you can do upper body one day, legs the next. But that doesn't work for me.
I think if you really want to get the most out of an intense workout, then it has to be preceded by a rest day. In other words, your best workouts come out when you are completely fresh.