The one thing I love about YUI is the nice "extend" function which makes everything sooooo much more beautiful and gives almost that nice C++ 'like' object oriented feeling.
Now, YUI is quite heavy, even with all the compression, etc. so I came up with a very light-weight extend implementation.
It's nothing new (or extremely brilliant), except that it works just like the "extend" from YUI; the source code can be found right here .
Public domain, enjoy :)