This isn't directly related to usability, but I just finished a date formatting function that I've been working on for a while.  It's based on PHP's date() function.  I frequently work with both javascript and PHP together, and I often find myself wanting PHP's date function in javascript, too, which doesn't have much in the way of date formatting itself.

Most of the code in there is stuff I find myself writing over and over again every time I work with dates in javascript, just bound together into one nice function.  If you think you would find it useful, too, go check out jsdate.

Edit:

I made some changes to one of the functions, and I found an error that had been preventing the minified version of the file from working.  So those should be better now.