lxr.php.net has been around for a while, but since I've found myself using it a lot recently, I want to point it out. It's such a quick way to find the particular file a built-in (or extension) PHP function is defined. Coupled with the
revision graph that
cvs.php.net can display, it is handy for tracking down exactly when a function was added or a bugfix was committed.
Thanks, lxr!