Proxies have been around for a while now, but remain poorly understood. I'll share some data on how they are used, what they are used for, and who uses them. To understand open proxies, I've scanned the Internet to track their prevalence, and monitored public statistics interfaces to understand the machines hosting them. ~220 TB of traffic flows through open proxies each day, making them one of the largest overlay networks in existence - especially since most operators appear to be running them unintentionally.