soft question - Most helpful math resources on the web

edit by jc: As of May 11, 2010, the work has been completed!

This is a reference that is not yet complete, but it should be very useful when it finally does arrive:

Digital Library of Mathematical Functions (DLMF)
(book and associated website;
will replace Abramowitz & Stegun's Handbook of Mathematical Functions)
NIST / Cambridge University Press
expected 2009/2010
http://dlmf.nist.gov/

This will contain diagrams, tables, properties of, principal values of, and relationships between many important mathematical functions. For example, the trigonometric and other elementary functions are described, with very many formulae relating them.

The Handbook is very good; the Digital Library will be even better.