As an app, you can enter a URL and immediately see the source code behind it. As an extension, it's even easier: just browse the web in Safari as you normally would, and if you want to view the source for your current page just tap the Action icon and choose View Source. You can then either copy the text to your pasteboard for use in other apps, or send it as an email.
To enable the Safari extension on iOS 8, launch Safari, tap the action button on the toolbar, scroll right to the end, then tap More. Make sure View Source is enabled, then position it wherever you want it in the list of Safari actions, and tap Done.
This app is optimised for all iOS 8-compatible devices, including the new iPhone 6 and iPhone 6 Plus.