Reviewed in A beautiful and improved Android experience
The only thing the app does is rounding the corners of your display. It's either totally useless or a neat looking effect. Looks nice on the Galaxy Nexus as it lets you match the rounded corners of the device with the screen.
Roundr is an application that rounds the corners of your screen! Functionality
Roundr features full settings which allow for adjusting the radius of the corners, as well as specifying which corners to enable or disable. With Roundr Extensions, you can customize the looks of the corners, and try out different designs!
Roundr Extensions add 10 new corner styles, blacklisting apps (to hide rounded corners on certain apps), and more! https://play.google.com/store/apps/details?id=mohammad.adib.roundr.extensions
Follow the development on XDA: http://forum.xda-developers.com/showthread.php?t=2234079