Magn QR Code for WP is a WordPress plugin that helps you to add QR Code or 2D Quick Response code into WordPress posts.
This plugin uses Google Chart API to generate the 2D QR Code image and then you can embed it into WP posts using a shortcode.
[qrcode href=""]
or you can use this in your template:
If you want to see this plugin in action you can visit
http://aplicaciones.org/
Install the plugin and then use the shortcode to embed the QR Code into your posts. Or you can customize the settings for example to change the default width and height.