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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.
A lot of folks who use ROM also use Hanami, and in some cases they'll also use one or more of the dry-rb libraries (I've used dry-monad before and had a fine time).
The Forme gem is proof that you can make a form builder where the ORM integration is loosely coupled. Rails maintainers could've spent the effort to decouple Rails' form builder, but I guess it's not a priority, since so few people use a different ORM with Rails.
NOTE:
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Hence, a higher number means a more popular project.