Sigmabox is a calculator webapp for mobile devices. As a rule, calculator apps tend to be either feature-rich or well-designed. Sigmabox tries to be both; it handles things like complex numbers, matrices, and statistical tests, but it arranges these features thoughtfully instead of cramming more buttons onto the screen as one might a physical plastic slab. Except as a blunt instrument, it functions pretty well as a day-to-day replacement for something like the TI-84. I say that, then sob gently, because nothing will ever replace the TI-84. Your grandchildren's grandchildren will be using the TI-84 to graph polynomials as their generation ships careen towards the Tau Ceti system.
Anyway, you can try it out right now from the link above. It's designed to be added as homescreen app on your newer iThing, but it usually also works on Chrome for Android. As of November 2014, you can also grab it from the App Store!