The Pacecar bookmarklet makes reading web pages with distracting graphics or a lot of crammed text easier by highlighting the section you're reading and dimming out the rest. Once you've saved the bookmarklet, you can visit any page, click to activate the bookmarklet, and get a mask attached to your mouse pointer to deprecate any visual information you want to avoid on the web page — helping you focus on the textual or graphical facts. It works best if you have a key command, mouse scroll wheel or other way of scanning down the page as you read. A simple contextual menu pops up if you right-click, allowing you to limit the width or expand the depth of the highlighted area. Double-clicking turns the tool on and off. Auto-scroll and auto-zoom on text would make the tool an even better accessibility aid. Check out this screencast of Pacecar in action.
The Pacecar bookmarklet currently only works with Firefox.