Legibility App is a tool for Typeface and UI Designers for evaluating typeface legibility under different viewing conditions. It’s a project by @viljamis and was originally inspired by a recent talk of his, called Typography for User Interfaces.
This tool allows you to simulate different viewing conditions by applying a level of filters on top of the text — like blur, overglow and pixelation. The app is currently in beta and works in desktop and iPad versions of Chrome, Opera and Safari.
Legibility App is completely free to use and experiment with. Future plans include the usage of opentype.js to access the letterforms directly for better simulation possibilities than what CSS only provides.
Please note that Legibility App is still in early development stage.