It is a popular library, but I still feel that it's quite underappreciated.
It is constantly living in the shadows of TensorFlow and PyTorch. MXNet has a clean API (via Gluon) and has nailed the hybrid eager/graph execution model better than either of its competitors.
I have come across multiple ML practitioners who have never heard of it.
I have recently had a lot of success with lucene/solr and wish I had used it for years!
Which have you found useful and want to help bring attention to?
Even if relatively large but you want to emphasize, give a shout!