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Example file: http://www.ejectmedia.net/examples/moo_textareas/
Mootools - Resizable Textareas
code
This is a simple script I made using the Mootools framework. You just link to this script and all the textarea elements in your page become resizable with a + and a - button below it.
Inside the script you may specify a maximum and a minimum size as well as a class name for textareas you'd like this script to skip over and not apply the rezizability to.
Example file: http://www.ejectmedia.net/examples/moo_textareas/