Since their are so many variables in your website, having an application do this for you isn't ideal. This is a job that should be left to the developer. The hardest part about building any website is not having any dependencies built-in, such as resolutions (i.e avoid width & height specifications), image/video sizes and additional software (Java, Flash, Silverlight, etc.). If you must do this, I would create a style sheet template for each resolution/device and detect the users platform (using PHP for example) and load the appropriate style from that. (See tools posted below)
Make sure you create a backup of your website before you begin the transformation into mobile compliant code. First make sure that your code is mobile browser compliant. Remove as many warnings & errors as possible.
Once your website is considered "Mobile OK" you can view your website using Adobe's Browser Lab and Gomez Mobile Validation. I think the Gomez Test Center looks more viable, however it requires registration. These tools basically allow you to see how your website will display on different devices and or different resolutions. (See more tools posted below)Ultimately, you'll never be able to satisfy every browser, device AND operating system whilst keeping your website design how you want it. Pick the devices and browsers you'd like to render best and specifically target them. If you plan on desktop, iPhone and Android users be the most commonly visited, pine to please those devices.I personally would lean towards different versions of your website. I would have a mobile ready template (/mobile) and your default template (root). The first script that runs could be the PHP file that determines which template or subsidiary template to load (i.e /mobile/iphone). Other articles & tools:
Mobile Meta Tags (Article)
Redesigning A Site For Mobile - What's Involved? (Article)
Why and How To Make Your Website Mobile (Phone) CompatiblePHP Script To Browser & Device
that covers pretty much everything you should know. Thank you Jeffery!