Sphinx: open source search

Sphinxsearch is a scalable open source search server that seamlessly integrates with Mysql. Documents are stored in mysql and the server creates its own external indices. The index and search performance are quite good (60+ MB/sec per server, 500+ queries/sec),  and the server allows to easily extend your own ranking functions. A sql-like language is supported for searching. Apparently, the largest cluster of sphinx servers is currently indexing 3billions of documents and it serves 50M queries per day. An interesting feature I am going to use is the ability to search all the geo-points within a fixed radius.