Top 5 knn-search open source projects

Non-Metric Space Library (NMSLIB): An efficient similarity search library and a toolkit for evaluation of k-NN methods for generic non-metric spaces.

A header-only C++ library for k nearest neighbor search with Eigen3.

C library for finding nearest (most similar) element in a set

Collections of vector search related libraries, service and research papers

vektonn.github.io/vektonn