The small form factor computer-on-board designs aren't really as new as Raspberry and others would like you to think. They've existed for ages, albeit for different purposes - mainly for embedded systems. Take this CM-X255 board I got my hands on recently:
Once again the photo quality seems to suck. That's what I get by not bothering to look for proper camera stand before taking photos. Lazy me... Although it does also have shiny conformal coating on it, making photography even more difficult.
The board is dated 2009, is about 67x44 mm in size, and the connectors are the thickest part of it. Currents ones come with ARM processor, SDRAM, some Flash storage, Ethernet, serial ports and loads of GPIO; this seems to have just about same stuff build on it. Current pricing seems to start from less than $50 per module (in volumes, I suspect). And these might be difficult to get as individual, too, unless you commit to buying at least 100 or so at a time.
Considering what it offers the packing density is pretty impressive. I tried to figure out how many layers it has, but that damn coating makes it fairly difficult. I think I counted at least 8 layers at board edge but can't exactly be sure. I'd say 8 is absolute minimum for density like this.
One major thing with these is that they require another board to properly operate; the idea is that you can design relatively simple application-specific board with whatever peripherals/IO you happen to need, and this board plugs directly in to that, taking care of the heavy lifting. Those board-to-board connectors aren't easy to work with either for hobbyist, being small, fine-pitch surface mount things with lots of pins. And for anything non-trivial multi-layer board is pretty much required, considering number of high-frequency signals on those connectors.
All in all, not as hobbyist-friendly like Raspberry and other more modern offerings, but good reminder that system-on-board devices did exist before them, too. They just were a bit more difficult to get into, at least for us common folk.
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