You can leave the TI graphing calculator at home thanks to this web-based TI-83 and TI-84 emulator. As with pretty much all emulators, this depends on a ROM image from the actual hardware to work.
For the lowest level courses, get the model you will eventually need in a later course (e.g., a student in MATH 1100 who will eventually take MATH 1160 should get a TI-83/84, but a student in MATH ...