Implantable Cardioverter/Defibrillator. Mine is implanted just over my left pectoral muscle and has two conductive leads (much like the ones kleinbl00 designed) that run to the heart to monitor rhythm and if necessary, deliver corrective electrical impulses to a heart that is misfiring. I'm on my second one. The first one I got when I was 14, was supposed to last until I was 18, and lasted until 19, almost 20. The one I have now is supposed to last well into my 30's which is weird to think about. I've only ever gotten one corrective shock but it saved my life. To answer the inevitable followup question, it hurts. It feels like getting kicked in the chest by a large ungulate. I especially like the tattoo thing because historically my family has been 100% against tattoos, piercings, basically any modification to 'The body Gawd gave you.'
My design work was for a company called InControl. I did a 2-quarter co-op with them, then went back to school. They asked me back (unprecedented in my school) but I needed to take statics, so passed. Three months later they were purchased by Guidant and 80% of their staff was laid off. I heard Guidant wanted their cardioverter technology. Six years after that, Guidant was purchased by Boston Scientific, thereby launching BSci into implantable cardio devices (previously, they had done stents, shunts and the like). It's probable that the device in your chest isn't just "much like" the ones I helped design, it's a direct descendant. You may very well have my labors inside your body.
I've heard some doozies. Friend of my mom has a birds nest of wire all through her pericardium because of some inept surgeon decades ago. That, combined with the video of the toothpaste-consistency, vibrant green goop that came out of an infected incision has given me a profound respect for good surgeons and post-op wound care.