DLMF
NIST has released a new version of one of our favorite books; the NIST Handbook of Mathematical Functions. As part of that, they have a new website called the Digital Library of Mathematical Functions. It has all of the great content from the book presented using MathML and lot of nice illustrations (including VRML & X3D plots). This is a great source for anyone writing mathematical software and it’s really nice to have it all online.