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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
So it looks like they're using the micronucleus bootloader for the linked example. That bootloader not only does the normal bootloader job, but also acts as a bitbanging USB device in order to communicate with the computer over USB. The USB bitbanging is a huge endeavor to undertake on top of writing a bootloader.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.