Stuff related to Indexing and Searching. Besides Spotlight, of course.
- Zoë - self-containted Java-based mail archiver and indexer. Requirers duplicating or migrating your mail store, eats RAM and disk like there's no tomorrow. Dated interface.
- mairix - simple, neat, effective if somewhat limited (does not index attachments)
- Namazu - C/Perl hybrid (format parsers are in Perl). Supports Japanese encodings.
- Lucene - Java-based, powers several different search applications and has spawned countless ports to other languages: