WP-LightForm, the prettiest WordPress contact form.
At the core of WP-LightForm is a XHTML Form element, to add or modify elements you can change the $formoutput string in lightform.php as you would a normal XHTML document. To modify the css styles to better suit your site you will need to change css/lightform.css.
Good question, my testing concludes that it definitely does work in version 2.5.1. Anything above and below that is untested, so if you decide to give this plugin a try on any other version please let me know how it went by leaving a comment.