This website offers a number of categorized links to various mathematical resources around the web:
http://www.intute.ac.uk/mathematics/
With links to hundreds of external resources on Applied Mathematics, Calculus, Math History, and other topics, there are plenty of e-books to download and websites to interact with.
It seems that the organization that runs the website, Intute, has run out of funding, and so no new content will be added, but the resources that are already there should remain.
This is another beautiful infographic from the New York Times that allows the user to interact with data from the 2010 U.S. Census.
http://projects.nytimes.com/census/2010/map
You can explore changes in population, density, distribution, and housing by looking at states and counties across the country.
This is a beautiful and interesting video that explores the construction of a hyperdodecahedron.
http://www.youtube.com/watch?v=MFXRRW9goTs
A hyperdodecahedron is like a four-dimensional version of the dodecahedron. It is also known as a 120-cell, as it is composed of 120 dodecahedrons. It has 720 pentagonal faces, 1200 edges, and 600 vertices.
“Beautiful Weather Graphs and Maps” are promised by WeatherSpark, and they deliver.
You can get local forecasts that show a number of projections bounded by highs and lows, or you can look at daily, monthly, or yearly trends that aggregate historical data.
In another section, a variety of interactive maps allow you to look at geographic trends in temperature, wind, humidity, and many other factors.
You can even look at long-term global temperature data!
A lot of information and representation to interact with and explore here.
Here’s another nice collection of free e-books on a wide range of topics.
http://www.e-booksdirectory.com/
The site has over 5,000 free e-books in subjects from Art, Computer Science, and Engineering. And the selections currently include about one thousand math e-books!