I think I first heard about LimeSDR-USB on The Amp Hour.
I was convinced to jump in and give it a go particularly because of the fully open-source nature of the platform - from hardware all the way to software, including FPGA code.
The unit is suited to research and development in a wide range of areas, but I suspect as I get more into this I will be mainly focused on areas such as test and measurement (e.g. spectrum analysis) and amateur radio.
As always, all notes, schematics and code are in the Little Electronics & Arduino Projects repo on GitHub
There are some good video resource, a good place to start is: What is the LimeSDR?