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I have run some tests in order to evaluate the legibility of the fonts included in the "Google font directory".
I have tried to read a text at about 50cm from my screen and evaluated the legibility of each of the 18 fonts from 9 to 18pt x-height with 3 levels : non-legible / difficult legibility / good legibility.
The tests have been run up on Firefox (last version) and 3 OS : Mac OS-X / Win Vista and Win XP.
The results of these tests are included in a pdf document, a kind of "Google fonts legibility cheat sheet".
If you are interested, you could download it for free (CC-licence).
Here is the link : Google fonts legibility.
I have written an article (fr) to explain my methodology on my blog. I will try to make an english translation when I would have some extra time.
I think this pdf could be useful for everyone who wanted to choose a legible display font in the Google font directory to be used on the web.
a french version of the pdf is available here : Google fonts legibility (fr)