OS X's Calculator application is something I basically ignore. Between Spotlight and Terminal's bc, I rarely if ever use it. So it was a pleasant and welcome surprise when TUAW reader Roberto Mateu gave us a heads up that Leopard's calculator was updated with better RPN support.
Calculator has done RPN for some time--if you're still running Tiger, you can see the old version by selecting View > RPN for any of the three calculator types. Under Tiger, the = button changed to Enter and that was about it. Leopard has updated the RPN calculator by adding multi-line entry so you can see your stack as it grows. It's a much nicer way to use RPN and TUAW heartily approves.