Maighstir: (The Kickstarter campaign I linked in the previous post has better detail)
The Jolla phone has contact points below the back cover, which can be used to provide power and two-way I2C communication to add-ons (the intention of the "The Other Half" system was to replace the back cover with another that has the function you want).
Unfortunately, it doesn't seem to have caught on very well (of course, the Jolla phone itself is fairly unknown), and the official TOH's only utilise the NFC sensor to identify themselves in order to authenticate for a download of a new theme (one for each available cover colour).
See
this image. Four contact points are close to the SIM card slot (right side) and two are on the left side. Also showing is the bundled "Snow White" TOH, with the NFC chip under the sticker (slightly lighter than the cover itself).
Project Ara has, also unfortunately, been cancelled only a few days ago, though some software bits are in use in devices by other manufacturers. There is the similar
PuzzlePhone, though it's much less modular and only has three standard pieces - camera/CPU/storage, buildingbase/screen, and battery/secondary features - rather than Ara's "add five cameras and three batteries if you wish" connect-anything-anywhere idea.
I just skimmed the Kickstarter page and their own website. It is a fairly interesting concept, but unfortunately unless it got backing from a big company and was available for the masses I don't see it catching on...
Which is a shame, because more interesting and different editions to the market are what we really need, but the fact that the Jolla is out of stock of everything doesn't bode well for it being a long term success, they obviously can't maintain manufacturing volume...
That's a real shame, I really liked the idea of a phone with customisable modules (want a better camera, don't get a new phone, just a new camera module). I did see that other one when looking for the name of the Ara, but it's far less interesting, though still possibly a step in the right direction.