I have done a number of challenges, but Ann Morgan, a journalist and author, took on the idea of reading one book from every single country in just one year. That's one book per 1.87 days!
I just think it's so great that she did it, had awesome interactions with people around the world about it, and I hope it inspires me to branch out a bit more in my reading. I do fairly well at reading books from outside the US, but I could definitely do better.
Here is an article about her experience
Here is the list of books she read by country
And here is a link to her blog, A Year of Reading the World.