The com.taylortext.ditasearch plugin for the DITA Open Toolkit generates a help index and search box in your HTML5 output.

Unique features include:

• DITA-aware indexing, so semantic markup like <indexterm> and <shortdesc> automatically increase the weight of terms in those elements.
• Uses a modified Porter2 stemming alorithm so variant forms of English words (running and ran, for example) are considered equivalent.
• Respects DITA <searchtitle> and @search markup.
• Includes a default synonym library that can be extended.
• Automatically adds the search box and calls the search Javascript from your HTML5 topics (you can turn this off with a parameter).
• Search results are listed immediately below the search box, with intuitive keyboard navigation (you don’t need to leave the current topic to view a page of search results).