Windows RT has been hacked to allow desktop apps to run on Microsoft's ARM-based OS. The trick circumvents a setting which usually restricts ARM-based apps to Microsoft's built-in ones, so that unsigned desktop apps can also be installed. The Verge explains:

Using a method to exploit the setting in memory, the vulnerability could allow Windows RT to run ARM compiled desktop apps. The revelation is a jailbreak, but no x86 desktop apps will run as they need to be compiled to ARM to work on Windows RT - making this exploit rather limited right now. The trick will need to be executed each time the operating system is booted up and will persist until it's reset.

So, not particularly useful right now, but expect to see third-party desktop apps from hackers and devs for Windows RT in the future. As Verge points out, though, it might not be such a great idea to take this route: the app restriction was put in place by Microsoft to extend battery life in RT tablets. Without the restriction in place, expect to see a slump in how long your power lasts. [On the Surface of Security via Verge]