Bluestar's WordPress Advanced Library
This library contains must-have functions which are not included by WordPress itself, but really should do so. Most of them are going to be pretty handful for you in case you're developing anything under WordPress.
How to use:
You can simply copy the "blue-wplib.php" file into your "wp-content\mu-plugins" directory - all of the defined helper functions are going to be available for use after it.
Have fun! :)