When the web was first born, the creators envisioned a place where all people would be able to access information equally. Some people are not able to get information the same way as most. For example, the blind have a difficult time using an image map, where clicking on different parts of an image link to different pages. Those who are color blind have a difficult time when told told to click on the blue arrow instead of the green, and those that are hearing-impaired don't receive the full benefit of a video broadcast. New initiatives are aiming to make the web more accessible for all.

Some of the ways to make your site more accessible are easy, if not so obvious how to do. Some examples:

Give the "alt" attribute a value for every image on the page
If images are used for navigation, have a redundant text link
Don't use color to convey information
Use text subtitles for all audio broadcasts

A separate text-only version of a page is not always the best solution either. Frequently, it is quite the opposite, as the text page doesn't get updated as frequently as the graphic content page, thus defeating the purpose.

The Web Accessibility Initiative group at the W3C have created a checklist of guidelines to follow which will give the most users improved access to your site.
http://www.w3.org/TR/WCAG10/full-checklist.html

Once a page is finished, and adheres to the guidelines, get it Bobby certified. This is a tool that will check your page against the guidelines and point out and inconsistencies.
http://www.cast.org/bobby/
If your page passes, (as this page does) it can proudly show the "Bobby Approved" icon.