Testicular torsion. It felt like I was being kicked in the balls every time my heart beat. It was my senior year of college and I was home for the weekend, but my parents were gone and I didn't think I could drive myself to the hospital. Luckily my uncle was around to take me. I dry heaved out the passenger window the entire drive there.
I had barely gotten into the room when the doctor came in and helped me strip down, then proceeded to do what he called the "open book maneuver", wherein he rotated my testicles to remove the twist. The relief was instantaneous, like my balls had been released from a vice grip. Luckily no permanent damage was done, but an ultrasound confirmed that I'd need surgery to prevent future torsions, which I had during spring break... yay.
Jarmer