My daughter collected tiny Legos - we called them Ones. Apparently the official name is 1x1. She gathered them and carried them around in a sock. Her brother was little, so the sock was meant to protect him from eating them.
My middle son loved cars - matchbox cars and hot wheel cars. He lined them up bumper to bumper, always in the same direction on any line he could find. I spent a lot my life stepping over lines of cars placed just so on the kitchen floor. He also loved bouncy balls, the little rubber kind that you get in machines.
My little guy, who just turned three, is obsessed with marbles. We did have the talk about his body openings not actually being pockets. I'm holding out hope that he will be the only three year old I've ever met that didn't cram a small object up his nose. Fingers crossed. He has no use for bouncy balls, no use for cars unless he's playing with his brother, no predilection to collect Lego pieces. He just loves his marbles (pronounced "narbles" by the three-year-old crowd).
He carries marbles everywhere. Now he has too many to carry in his hand or in his shirt. So he uses random plastic food storage containers - like GladWare - to carry around his preciouses.
I don't remember having a collection obsession when I was small, but I do remember circling all the two letter words in every book I could find and having a strong need as an older child to be surrounded by books.
What was your childhood collection obsession?