The Encyclopedia of Mathematics wiki is an open access resource designed specifically for the mathematics community. The original articles are from the online Encyclopaedia of Mathematics, published by Kluwer Academic Publishers in 2002. With more than 8,000 entries, illuminating nearly 50,000 notions in mathematics, the Encyclopaedia of Mathematics was the most up-to-date graduate-level reference work in the field of mathematics.
The Encyclopedia Britannica is an excellent resource for general information on a subject. It can be used for background reading for research, or just for your own interest.
Gapfinder.org is a non-profit venture promoting sustainable global development and achievement of the United Nations Millennium Development Goals. Gapfinder seeks to unveil the beauty of statistical time series by converting boring numbers into enjoyable, animated and interactive graphics. The website has a large range of data sets which can be displayed in graphic form or downloaded as a file.
MIT Videos on Mathematics are available free on the internet. Free lecture notes, exams, and videos from MIT. No registration required. OCW is open and available to the world and is a permanent MIT activity.