Since the App Store launched, there hasn't been much that has really tempted me to jailbreak my iPhone. I don't think Mini vMac for iPhone actually will either, but just that fact that it exists at all is a trip for the nostalgic Macheads out there. Mini vMac is an open source Mac Plus emulator that has now been ported to the iPhone (a regular OS X version exists as well).
It emulates a 4MB Mac Plus; as the video above shows, it allows you to scroll to show the full display and even load Mac OS disk images and run classic Mac software. It's safe to say that this will never make to the App Store because it violates Apple's rules against emulators.
Mini vMac for iPhone requires a jailbroken iPhone, and can be loaded by adding the namedfork.net repository to the Cydia installer as described on the main page. You'll also have to round up a Mac Plus ROM image and disk images of any applications you want to run.