iOS (jailbroken) Whited00r is the easiest way to get the newest version of iOS on an unsupported phone, and it has just been updated with iOS 7 features. It's essentially a very complicated theme, and it now lets you get iOS 7 on older devices like the iPhone 3G.
All of the big features of iOS 7 are included here. You'll get the new look, but you'll also see the new notifications, the improved camera app, the new multitasking function, Control Center and more. You can find the downloads and installation instructions for your device at the link below.