During the '80s and early '90s, it wasn't unusual for console makers to equip their machines with expansion ports.
With an eye on future developments, these ports (in theory) would allow accessories and bolt-on devices to augment the power of the base system; with the Super Famicom (Satellaview) and Mega Drive (Mega CD) this came to pass, but in the case of the NES, the 15-pin Expansion Port on its underbelly was never used during its lifespan.
It would presumably have allowed the console to connect the Famicom Disk System, but that particular add-on was never released outside of Japan.
Fast forward to the present day, and hacker and modder RetroTime is looking to make use of the port via the NES Hub, a new homebrew accessory which will offer a wide range of applications.
The NES Hub is a plug-and-play solution which will pave the way for things like 4-player Bluetooth support (making the Four Score accessory redundant), SNES controller support, Famicom accessory support and the ability to run various third-party devices and add-ons.
The most exciting development is the potential to integrate the Muramasa Disk System Cartridge and Disk EXP Adapter, a previous example of a device which makes use of the Expansion Port – in this case, to play Famicom Disk System games on the NES.
RetroTime is looking for more beta testers to continue the development of NES Hub and admits that it has only been tested on a PAL NES console so far. The price is also unknown and "depends a lot on the production cost of the connector."
Let us know if this is something you'd like for your NES by posting a comment below.