The following radios and radio boards are compatible with OpenTX.

Tray version of Taranis Plus with:
Color OLED top mounted display for basic telemetry data
Quality 4 bearing gimbals
Bluetooth support
2 additional sliders
Safe start and safe shutdown power switch
Haptic vibration
Improved audio circuits

New version of Taranis with:
Haptic vibration
Multi colored LCD background light
Improved LCD display
Improved audio circuits

ARM Cortex M3 32-bit 120MHz
212x64 LCD with gray scales
60 models + infinite on SD
9 flight modes
Telemetry
Advanced audio functions (voice files from SD, vario, ...)
SD data logger

ARM Cortex M3 32-bit 36MHz
60 models + infinite on SD
9 flight modes
Telemetry Interface through Futaba trainer jack
Advanced audio functions (voice files from SD, vario, ...)
SD data logger

Arduino (or clone) MEGA2560 16MHz
30 models + infinite on SD
6 flight modes
2 rotary encoders (one can be used for navigation)
Telemetry
SD data logger

ATmega128 8-bit 16MHz
30 models
5 flight modes

PLEASE NOTE that Turnigy 9X's sold by Hobbyking since summer 2016 use a "clone" processor that does not work correctly with existing 9X firmwares. DO NOT flash OpenTX to it as it will not work reliably, and there is no known source for the original firmware to restore it.

ATmega64 or ATmega128, 8-bit 16MHz
16 models
5 flight modes
A lot of mods supported (audio, voice, telemetry, rotary encoder navigation, ...)

ARM Cortex M3 32-bit 36MHz
60 models + infinite on SD
9 flight modes
Rotary encoder navigation
Telemetry
Advanced audio functions (voice files from SD, vario, ...)
SD data logger

ATmega2560 8-bit 16MHz
30 models + infinite on SD
6 flight modes
2 rotary encoders (one can be used for navigation)
Telemetry
SD data logger