Most recent iPhone apps come equipped with Retina display graphics for use on the iPhone 4; despite this the iPad will run them at double the size, the apps look pretty pixelated. No more!, RetinaPad is a jailbreak extension for iPad that lets any screen-doubled graphics use the retina version, significantly improving the resolution of most any small screen app.
The $3 app is incredibly simple to use too, just buy the app and install through Cydia using a repository of your choice. Once RetinaPad is installed you'll need to restart Springboard and launch any iPhone app you want to enable. When you launch it for the first time after installation, RetinaPad will ask you if you want to let it run regularly or use graphics designed for the retina display. If you choose the retina display, you'll need to close the app and then quit it via the multitasking bar.
When you launch the app again, it will use any retina graphics available. This may not be much more than standard user interface elements (e.g. the keyboard), but it makes a huge difference with any apps that support the retina display but not the iPad (like is the case with many games).
So if you're an iPad user that still relies on iPhone apps that haven't got iPad versions yet, and have a jailbroken iPad, then RetinaPad is certainly worth checking out.