Xapian is excellent. It's an embedded C++ library, and there are bindings for most languages. It's well-designed but fairly basic. Since it's a library, you'll

Xapian is excellent. It's an embedded C++ library, and there are bindings for most languages. It's well-designed but fairly basic. Since it's a library, you'll have to write the server part yourself. (It has a server called Omega, but I've never used it.) Indexing is slower than Lucene, though, but it's not too bad. The only significant problem I found with it the last time I was using it (which was 5+ years ago) was that writers occasionally would cause queries to abort, and you had to restart them, a cycle which could lead to long query times if you had lots of write churn.