You might want to build custom indices of documents for many reasons. A widely cited one is to supply search functionality to a web site, but you also may want to index your e-mail or technical documents.
Swish-e is a fast, flexible, and free open source system for indexing collections of Web pages or other files. Swish-e can index plain text, e-mail, PDF, HTML, XML, Microsoft Word/PowerPoint/Excel and just about any file that can be converted to XML or HTML text. Swish-e is also often used to supplement databases like the MySQL DBMS for very fast full-text searching.