According to Wikipedia, "accessibility is a general term used to describe the degree to which a system is usable by as many people as possible without modification. … One meaning of accessibility specifically focuses on people with disabilities and their use of assistive devices such as screen-reading web browsers." Many people, myself included, do not spend much time thinking about accessibility; however, we probably should.
Several recent announcements by governments standardizing on open source software have generated strong reactions from accessibility advocates. The advocates highlight how many open source products are not compatible with screen readers and other devices used by disabled people who rely on them in order to function effectively at work. Many people are concerned that a move to open source software could result in lost jobs for disabled people who rely on additional accessibility technology.
Some open source products strive to meet accessibility requirements; for example, OpenOffice.org works with the JAWS screen reader. As an example of an accessibility issue, one Linux user in Italy had to find someone to install Linux for him and install the driver for his Braille terminal before he could productively use the Linux operating system. He also struggled with effectively using the Linux documentation, which made ample use of screenshots.
We need to work within the open source community to make sure that more products accommodate accessibility technology. I also encourage people who use these accessibility devices to get involved in open source communities to help increase accessibility of open source software.
This NewsForge article provides many more details and links if you want to learn more about accessibility for open source software.