Most documentation tools let you generate an index of terms in the document, but often it is useful to be able to generate indices for special purposes. For example if you are documenting software and index to functions, an index to types etc may be useful.
Flexndex is a flexible tool to generate multiple indices as document internal links, making it usable with most XML based document formats, eg xhtml, docbook or Open Document Format.
For more information see the full manual.