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Purpur
Purpur is a drop-in replacement for Paper servers designed for configurability, and new fun and exciting gameplay features.
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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.
Personally, I just run a PurPur server locally on my machine and connect to it with my client using "localhost". Not only lets you have any plugins you want, but provides very useful experience running Bukkit servers.
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.