We reckon the calculator in Windows 7 is pretty neat, but it's not to everyone's taste. If you prefer your calculations displayed as you make them, Moffsoft FreeCalc is a pretty handy option.
Beyond that feature, the calculator has pretty much all the standard options you'd expect. There's a shareware version which offers exporting features and additional options, but for most calculator needs the basic option is pretty good. For another approach to calculation, check out the replica iPhone calculator.