The  page  allows teachers and students to download a free six month subscription to either TI-Nspire, or TI-84 Plus CE Smartview, or TI-30XS/TI-34 Smartview.

As well the TI-Nspire iPad app is available until the end of April for free on the Apple app store.

Further details are available directly on the page,

For Apple devices: 

A free ti 84 emulator for Apple devices (not sure about android)