For Vector Search the top 2 are: Meta’s FAISS and (my) Unum’s USearch.
Lucene powers Elastic, Solr, MongoDB Atlas, AWS OpenSearch, Azure Cognitive Search.
USearch powers ClickHouse, DuckDB, YugaByte, TiDB, ScyllaDB, MemGraph, KuzuDB, Lantern, and a few big closed source names that don’t mention it, as far as I know.
FAISS has the highest usage among Python developers, but if you are indexing large collections you should consider alternatives.
I went looking around last year and couldn’t really find many options, but I might have been looking in the wrong places.