The 2005 edition of the freely-downloadable 53-page guide to introduce software developers to the issues regarding accessible software. Provides an general overview of the access problems posed by various disabilities, and gives guideline advice about how to deal with them.
The document starts by defining the needs of those requiring accessibility, and outlines the benefits of providing it. Three principles of accessible software are presented, along with case studies of how developers can design their software accordingly.
The appendices include a good listing of resources, centres and organisations that can provide further advice or information on accessibility.
This 512Kb pdf document can be downloaded here.