This is mine, hands down, by far.
It seems to be part of an automotive brake pad, with no friction material attached. It's about 5" long ear to ear, and 3/8" to 1/4" thick. The silvery, slightly chewed bit was sticking pretty far out of the tire, and I had been driving on it for a long time by the look of it (so THAT'S where the odd feeling that I had a wheel out of balance was coming from.)
Half the tread lug was ripped off, with this replacing the missing half lug. I could peel back the "flap" of the remaining portion of the lug to expose the first hole far enough to see that it went on for a little way inside the tire. I tried everything I could think of to get this out when I saw it, and other than busted knuckles and a broken screwdriver, I didn't have any luck. I had a feeling it wasn't going to puncture the tire, which was still holding a perfect 105 pounds, so I drove it another 30 miles home to let the truck's owner have a whack at it.
After he got some busted knuckles and a broken screwdriver of his own, he cut away enough tread to get a firm hold on the hole, and put a big hook through it, which he attached to his pickup, and proceded to extract this behemoth thing from the tire.
Amazingly, it ran all week, and seems only a little worse for wear. I'd rather replace the tire at this point with its missing lug, but it's not my truck, and I'll try to keep running the tire until it reaches the end of its life in a month or two. (If it doesn't blow. Who knows. It's the rear outside drive, so hopefully it will only take a mudflap with it if it does.)
I've seen a lot of stuff in tires, and I thought the screwdriver that wound up going through my tire handle first would win the prize, but we have a new champion. Not only is this the biggest piece of metal I've ever picked up with a tire, but it miraculously avoided ripping a huge gash, and causing a catastrophic deflation. This could easily have been a pile of gator bits.
I think I picked this up when I took the shoulder really hard in a panic brake check. I wouldn't have ass-ended the truck in front of me, but I would rather see I was going to miss him by 3' from the shoulder, instead of where it would have hurt a lot more if I hadn't pulled it off.