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Marlin
Marlin is an optimized firmware for RepRap 3D printers based on the Arduino platform. Many commercial 3D printers come with Marlin installed. Check with your vendor if you need source code for your specific machine.
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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.
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Marlin is a solid default option. It will run on basically anything and has incredible customizability and configurability, though as u/tdetrina noted, the vast majority of the configuration gets baked into the firmware at compile time, so it can be very clunky to fool around with different configurations.
Klipper is the hot new kid on the block. It offloads most of the control logic to a companion board such as a Raspberry Pi, allowing it to run advanced features such as input shaping and network connectivity with even a basic controller board. The setup process is considerably more complex, but after installation, making any change to a Klipper machine is as simple as editing a configuration file and rebooting the machine.
RepRapFirmware is primarily designed for Duet3D's line of extremely reputable (but pricey) control boards, but it can also be run on many other boards as well. It features a lot of the same advanced features as Klipper without requiring a companion, but it requires a controller board with considerably more oomph than the others.