gistfile1.php

<?php
/*
Plugin Name: Pop Up Once
Plugin URI: https://gist.github.com/4394411
Description: Quick plugin to create a modal popup form
Author: Aaron Brazell
Author URI: http://technosailor.com
Version: 1.0
*/

class AB_Popup_Once {

	public function __construct()
	{
		$this->hooks();
	}

	public function hooks() {
		add_action( 'wp_head', array( $this, 'popup' ) );
		add_action( 'wp_enqueue_scripts', array( $this, 'js' ) );
		add_action( 'wp_head', array( $this, 'css' ) );
	}

	public function js()
	{
		wp_enqueue_script('jquery');
	}
	public function css()
	{
		?>
		<style>
		.ab-popup-layer { position: absolute; top: 100px; left: 100px; background: #fff; height: 300px; width: 300px; border: 1px solid #000;}
		</style>
		<?php
	}

	public function get_form_html()
	{
		// Do your form HTML here

		$html .= '<div class="ab-popup-layer">';
		$html .= '<form action"" method="post">';
		$html .= ' <label for="ab-form-field">Field 1</label>';
		$html .= ' <input type="text" name="ab-form-field" id="ab-form-field" />';
		$html .= '</form>';
		$html .= '</label>';

		return $html;
	}

	public function popup()
	{
		if( isset( $_COOKIE['ab_popup_done'] ) )
			return false;

		setcookie( 'ab_popup_done', 'yes', time() + (60*60*24*365) );

		$html = $this->get_form_html();
		?>
		<script>
		jQuery(document).ready(function() {
			jQuery('body').append('<?php echo $html ?>');
		});
		</script>
		<?php
	}
}

$ab_popup_once = new AB_Popup_Once;