The application requires a rooted Galaxy Nexus, Nexus 4 or Nexus 10, and will let you unlock and relock the bootloader as many times as you like. Since this approach doesn’t open up the miscellaneous partition on your Android to code modifications, and since the app itself remains under your control, using BootUnlocker makes sure that you retain a certain measure of security when it comes to bootloader-level vulnerabilities.
BootUnlocker for Nexus Devices is available over at the Play Store as a free download, and will not work on Nexus 7.