A general release announcement of jQuery 1.4, provided by the official jQuery 1.4 author, The 14 Days of jQuery.
My quick post about jQuery 1.4 almost right after it was released.
Good insight in new features of jQuery 1.4, provided by NetTuts+.
Collection of resources about jQuery 1.4, provided by W3 Avenue.
And another, useful article about jQuery 1.4, provided by tripwire magazine.
General Web Development
A good collection of icons. Far better than usual, in my opinion.
HTML and CSS
Nice and comprehensive article about CSS3 Transitions.
jQuery Lint is a script written by James Padolsey, which evaluates jQuery code and reports any errors or incorrect usage.
ie-css3.js allows Internet Explorer to identify CSS3 pseudo selectors and render any style rules defined with them. This project is still in progress, but it looks very promising.