Well, for a very basic scientific calculator, just use the OSX included calculator, and switch it to the "advanced" mode. Combined with the paper tape to make things easier to see (display that, too), you have a basic set of exponential, trigonometric, and algebraic functions.
For more power, there are many options, including emulators for beefy scientific calculators. Go to VersionTracker.com and do a search on "calculator" for dozens of options.