Further to the other answers given, another manufacturer that has extended the piano’s 88 key range more than any other is Stuart & Sons, an Australian builder of exquisite hand made pianos.
Stuart & Sons produce pianos with 97 and 102 keys. Their 97 key pianos extend the range by 5 keys at the treble end to F8, and 4 keys at the bass end to F0, a full 8 octaves. Their 102 key pianos extend the bass range by a further 5 keys to C0.
The Bosendorfer Imperial Grand extends only the bass end by 9 keys to C0 for a total of 97 keys, also achieving 8 full octaves but in a lower range than the Stuart & Sons 97 key piano. The Bosendorfer also colours the extended “white” keys black, to help prevent confusion for pianists more familiar with 88 key pianos.
The fundamental frequencies of a standard 88 key piano range from 27.5Hz (A0) to 4186Hz (C8). The Stuart & Sons 102 key piano extends this to 16.35Hz to 5587.7Hz, a not insignificant increase.