Below are a set of alternatives to a TI-84 graphing calculator. Whether you have a computer, iPhone, iPad, or Android device, there are many options for you.
Web-based
Desmos – this can do some pretty cool things. Check it out.
Windows PC Computer
Follow these instructions to install Wabbit-emu, a TI-84 calculator emulator. (Instructions courtesy of Brockport Central School District).
iPhone/iPad/iPod
None of these have been tested by me personally, but the reviews are solid:
Free Graphing Calculator (free)
Quick Graph: Your Scientific Graphing Calculator (free)
Android
Algeo Graphing Calculator (free)
Wabbit Emu (app) TI-84+ emulator, requires a calculator ROM file
Andie Graph (another TI-83 emulator, but requires some work to set up)