Gas masks. I'm obsessed with them. They come in so many different types, shapes and sizes - each one is unique. The lenses, the filters, everything about them is so intriguing. An old Soviet Union CCCP GP-5 mask looks the coolest but isn't as effective as some newer ones because they are outdated, but it's still a great addition to the collection. It's a pretty unusual obsession and that's why I like it.