Elasticsearch is a distributed search engine supporting various data types, including vectors, and provides scalable vector search capabilities, making it a popular choice for modern AI-powered applications. It can be extended with the k-NN plugin to provide scalable vector search using HNSW and Lucene, enabling hybrid semantic and keyword search capabilities.

Orama is a lightweight search engine that supports vector and hybrid search functionalities, suitable for browser, server, or edge environments.

Solr is a mature open-source search engine that has incorporated vector search capabilities, making it relevant for enterprises looking to implement vector-based search alongside traditional keyword search.

Typesense is an open-source search engine that supports hybrid search, including vector search capabilities, providing an alternative to proprietary vector search solutions.

Arroy is an open-source library for efficient similarity search and management of vector embeddings, useful in vector database systems.

Crate is an open-source distributed SQL database with support for vector data types and vector search, suitable for AI-driven applications.