Mac OS X Calculator
I have never seen a default calculator included with the OS that I like. For that matter, I have never seen a GUI calculator that I like with the exception of HP emulators (which are too clunky) and the RPN Calculator Dashboard Widget (which is great but dashboard currently means heavy swapping because of the RAM shortage). I have been pleasantly surprised once again by my iBook. Apple's Calculator is a marvel to behold.
It starts out as unassuming as you ever thought the default calculator would be.
Then you explore the menu and find that not only can you use RPN, but you can do scientific mode. Neat, but not new. (The RPN bit was exciting though)
Wait! What is this Progammer view all about? Oh wow. Easy base conversion, 64-bit binary, 2's complement, every operation you ever needed to make your CS and EE homework a piece of cake. I'm sold!
Ok, one thing does bug me. RPN is hard to work with without a stack. Sorry, the paper tape doesn't cut it (it might, if it responded to enter). But all in all the best designed software Calculator I have ever seen. It even beats out bc and dc thanks to the programmer feature. Still, there's nothing like my HP 48G...