Dowser is a Web research and archiving tool that clusters results from search engines, associates words that appear in previous searches, and keeps a local cache of all the results you click on in a searchable database along with summaries and links to related information. It helps you to keep track of what you find, with no advertising.
nmzmail is a tool to use the namzu2 search engine from within the Mutt email reader to search email stored in maildir folders. Based on the result of the namazu query, nmzmail generates a maildir folder containing symbolic links to the messages matching the query. A simple Mutt macro makes it easy to use from within Mutt.
Terrier is software for the rapid development of Web, intranet, and desktop search engines. More generally, it is a modular platform for building large-scale information retrieval applications, providing indexing and probabilistic retrieval functionalities. It comes with a desktop search application.
Hyper Estraier is a full-text search system. It can be used as a Web search engine, mailbox searching, etc. It features high performance searching, high scalability of target documents, a perfect recall ratio by the N-gram method, phrase searching, attribute searching, and similarity searching. Multilingualism is supported with Unicode. It is independent of file format and repository, and has a simple and powerful API.