But, now there is some serious competition in the form of Asustor. As the name perhaps suggests, this is an offshoot of Asus, the Taiwanese company that happens to be the World's 5th largest supplier of personal computers. Asustor was setup by ex-employees of QNAP and it shows in the products, which are like QNAP only "more so".
The hardware is superb. Whereas many low-end NAS boxes are made of plastic (albeit high quality plastic in the case of Synology and QNAP), all of the Asustor units have metal cases. And whereas many vendors use ARM processors, similar to what you would find in a mobile phone, Asustor use Intel x86/x64 chips, the same as you would find in a laptop or other computer.
But as with 'the big two', it is the software that really makes the difference. The Asustor operating system or firmware, known as ADM, looks uncannily like iOS on an iPad. Although it is accessed using a browser, it behaves like Windows or Mac: there is a Desktop, icons, windows, things can be dragged around and so on. But it is not just a pretty face - there is real functionality behind it, just as with Synology's DSM and QNAP's QTS.
Asustor seem to be committed to the market. ADM is now up to version 2.3 (with 2.4 in beta) and they currently offer around 2 dozen different models, aimed at everyone from home users through to large companies.
So, welcome Asustor!