The next Yahoo hackday will be on May 9th and 10th in Covent Garden. I've registered my interest. You should too.
The justification behind YSlow. If you've heard Nate Koechley speak, some of this will be familiar to you. It's all solid advice as far as I can tell.
The YUI folks have released an add-on for Firebug that will analyse your pages and suggest ways of speeding it up.
YUI Version 2.2.0 Released: Browser History Manager, DataTable, and Button Components, New Versioning, and More » Yahoo! User Interface Blog
Roll up and get it: hot off the presses; the new version of the Yahoo User Interface library. Happy birthday, YUI.
A handy tool for creating grids using Yahoo's CSS.
YUI’s Matt Sweeney and Hands-on YUI Workshops at Yahoo! Developer Day/Hack Day September 29-30 in Sunnyvale » Yahoo! User Interface Blog
Yahoo is opening up Hack Day to the masses. If you're anywhere near Sunnyvale on September 29th, this should be fun.