Simple Square Icon Pack

Note: This is not an standalone app, you will need a custom launcher that allows such customization.

Use any of the following custom launchers:
Nova Launcher
Apex Launcher
Go Launcher EX
Holo Launcher
ADW.Launcher

Also:
Launchy widget
Beautiful Icon Styler
Themer

Pick "Simple Square" from your launcher's icon pack settings, or edit each icon if you want.