Here’s an example on how to create a plugin that allows you to insert iframes using shortcodes. Regular iframe tags can be inserted via the HTML editor in WordPress, but the example below will give you a hint on what it can be used for.

<?php
/*
Plugin Name: Iframe Shortcode Plugin
Plugin URI: http://www.guidecloud.com/2010/07/14/writing-your-own-iframe-shortcode-plugin-for-wordpress/
Description: Add iframes to Posts and Pages within WordPress.
Author: GuideCloud.com
Version: 1.0
Author URI: http://www.guidecloud.com/
License: GPL2
*/

class guidecloud_iframe_shortcode {
    function shortcode($atts, $content=null) {
		extract(shortcode_atts(array(
			'url' 	=> '',
			'scrolling' 	=> 'no',
			'width' 	=> '100%',
			'height' 	=> '500',
			'frameborder' 	=> '0',
			'marginheight' 	=> '0',
		), $atts));

		if (empty($url)) return '<!-- Iframe: You did not enter a valid URL -->';

	return '<iframe src="'.$url.'" title="" scrolling="'.$scrolling.'" width="'.$width.'" height="'.$height.'" frameborder="'.$frameborder.'" marginheight="'.$marginheight.'"><a href="'.$url.'" target="_blank">'.$url.'</a></iframe>';
    }
}

add_shortcode('iframe', array('guidecloud_iframe_shortcode', 'shortcode'));
?>

Create a new file called iframe-shortcode.php, paste the code above and upload it to your wp-content/plugins/ directory. Go to “Plugins” in your WordPress administration and activate the plugin.

Once the plugin has been activated you can place iframe shortcodes, like the example below, into your WordPress posts.

[iframe url="http://www.linktoiframe.com/iframepage/" height="500" width="100%" scrolling="no" frameborder="0" marginheight="0"]

You can also download the plugin here:
WordPress Iframe Shortcode Plugin

More reading:

• Writing a plugin – WordPress Codex
• WordPres Shortcode API