I used to have the Decreased Productivity browser extension (Firefox only) installed, but when looking it up now, it seems that the project has been abandoned (no update since 2010) and it's no longer compatible with recent versions of Firefox.
The main thing that the extension does is to:
- hide or dim images (showing them with a simple mouseover)
- reformat text
Here's an excerpt of an article on Lifehacker about Decreased Productivity:
Decreased Productivity adds a small toolset to your Firefox browser that, once activated, drops a style sheet over the web page you're looking at that washes out the images and converts all the text to a plain, boring font. Anyone who glances at your screen as they walk by won't have a clue what web sites you're visiting and when the coast is clear just hover your mouse over the image place holders to see them again.
If possible, I'd like to find an extension with a similar set of features, ideally not limited to a single browser.