* FPGAs, and their associated FOSS stack to use them.
I haven't been able to really get into FPGAs but I'm optimistic about them. They're pretty clunky right now but I'm hoping they'll just get easier and more accessible.
If we want computation that we're able to verify as being secure, FPGAs are the closest I see us getting to it. There's also applications in terms of parallel computation that they can do over more traditional approaches.
This might go by the way of Transmeta or always remain niche but it seems like they have a lot of potential.
* Open Source Hardware, specifically relating to robotics
Electronics production is becoming cheaper and easier. Open source hardware has the potential to become as ubiquitous as open source software.
Electronics hardware is still way harder than it needs to be, so the progress is slow, but if we get within range of having an iteration cycle in electronics that's as fast as software development, we'll see spectacular innovation. Robotics especially, as that's a kind of straight forward physical manifestation of electronics that has a potentially large market.
There's a $5k fiber laser that can ablate copper. This could potentially fuel the next round of cottage industry board fab houses (in the US and other non-Chinese countries) and enable rapid turn around time. I wish I could justify the $5k to play around with it but it's just outside of my price range.
* Solar
I'm not really sure if this is 'under-the-radar' but for the first time, solar has become cheaper than coal. This means besides giving a moral incentive for people to use solar, there's now an economic one, which means the transition will most likely be broad.
Coupled with battery technology advances, this could have drastic impacts on the ubiquity and cost of power. I wonder if we'll see a kind of "energy internet" where people will create their own distributed electrified infrastructure.