The Neo900 is the spritual successor of the Nokia N900. The new circuit board can be placed into an existing N900 for better specs (faster CPU, more RAM, LTE modem) than the original device while still maintaining fremantle (maemo 5) backwards compatibility. Alternatively, a fully assembled phone can be purchased as well. The Neo900 will be fully operational without any binary blob running on the main CPU. While the modem still requires a non-free firmware, it is completely decoupled from the rest of the device (think of a LTE usb stick you put in your laptop) and can reliably be monitored or switched off by the operating system.
You can follow the development of the project in the maemo forum, read about the specs of the device or consult the FAQ