The advanced help module allows developers to store their help documentation within their module directory in pure HTML files. These files can be easily translated by copying them into the right translations directory. The help system can appear in popups.
Advanced Help ties into Drupal's search system and is fully indexed. The entire contents can be searched for keywords. Help files can be placed in a hierarchy, allowing for top down navigation.
By itself, this module doesn't do much; it supports other modules, but it so very well. Views makes extensive use of Advanced Help to provide in-context help and detailed documentation.