iOS has a growing developer community and while it's very far from the jungle of custom ROMs available for Android, here's a start. The Whited00r firmware brings iOS 7 look and feel to devices that are no longer supported by Apple.
Note that despite the looks, Whited00r is actually based on iOS 3.x. That said it includes multitasking, control center, Siri, lockscreen notifications and new camera app with video recording.
Both the official App Store and Cydia are available, though keep in mind that many apps will not run on the older OS. That's where the AppTimeMachine feature steps in and gives you access to applications that no longer support iOS 3.x.
The custom ROM is currently at version 7.1 and will get over-the-air updates via Cydia. The installation process can be done from a Windows PC or a Mac. For locked iPhones there is a special version of the software that will unlock the device first.
This probably goes without saying, but Whited00r is anything but official so installing this is done at your own risk.