Color Enhancer extension helps people with partial color blindness distinguish between shades of color when browsing the web.
Animated gifs and other animated content can be distracting for computer users with cognitive disabilities, The Animation Policy extension will allow users to control how animated images work in their browsers. Now you can prevent that annoying gif from continuously playing over and over again.
PC Magazine published an article that gives a little bit more information about both extensions.
Here are few more that you might find useful as well.
WAVE Evaluation Tool
This tool evaluates webpages for possible accessibility issues. This could be useful for special ed or general ed teachers looking for pages for a class assignment.
This is a screen reader for people who are visually impaired. It will read webpage text in a synthesized voice. Apparently, it is built into the Chrome OS on Chromebooks, but this extension will allow you to use it on your PC or Mac as well. Google has an interactive tutorial for learning how to use it.
ChromeVis will allow users with low vision to zoom in on or change the color of selected text. Here is the user manual for more information.
Though this might be considered more of a reference or search tool, this extension from Google lets you click on a word to get a definition from Google's dictionary. You can even click on a speaker icon to hear the pronunciation of the word.