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PiTubeDirect
Bare-metal Raspberry Pi project that attaches to the Acorn TUBE interface and emulates many BBC Micro Co Processors
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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.
There's a similar project for the Acorn BBC Micro called PiTubeDirect, which allows the Pi to emulate several different CPUs, while connected to the BBC Micro's "tube" second processor slot.
https://github.com/hoglet67/PiTubeDirect
(The BBC Micro itself was quite remarkable for supporting multiple processors, which did not have to be the same architecture as the host 6502.)
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.