CERN's Large Hadron Collider has been under construction for the last 10 years. It will be the world's largest and fastest particle accelerator when it comes on line next month, moving protons at 99.99% the speed of light.
Not up to speed on particle physics? Need a refresher? Take five minutes to watch this "Large Hadron Rap," and you'll know as much as I do:
Want to know more? Wikipedia has a nice summary.