The National Institute of Standards and Technology (NIST) has put online a preview of its famous Digital Library of Mathematical Functions (DLMF).
From the Preface:
Abramowitz and Stegun’s Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables is being completely rewritten with regard to the needs of today. The new DLMF (Digital Library of Mathematical Functions) will appear in a hardcover edition and as a free electronic publication on the World Wide Web. The authors will review the relevant published literature and produce approximately twice the number of formulas that were contained in the original Handboook. The DLMF will make full use of advanced communications and computational resources to present downloadable math data, manipulable graphs, tables of numerical values, and math-aware search. The authoritative status of the existing Handbook, and its orientation toward applications in science, statistics, engineering and computation, will be preserved.
Thus the utilitarian value of the Handbook will be extended far beyond its original scope and the traditional limitations of printed media. The term digital library has gained acceptance for this kind of information resource, and our choice of project title reflects our hope that the NIST DLMF will be a vehicle for revolutionizing the way applicable mathematics in general is practiced and delivered.
See also NIST Releases Preview Of Much-anticipated Online Mathematics Reference, ScienceDaily (June 27, 2008).