A new tutorial on PHPBuilder.com gives a quick look at creating a form combining HTML and PHP that can send out emails to the address of choice.
Like any email form, the original form itself is quite simple. No great shakes here, just a standard HTML form with a POST method and the action pointing to the PHP page that will process the form results.
Included in the short tutorial are the HTML for the form, the PHP to process the email information from the form and the code to send it all out.
It's also important to note that they defined the email address to send it to in the PHP code handling the POST request. If you allow your user to define the email address, you're begging for spammers to come around and abuse it.