Here's a list of applications written in rust: https://github.com/rust-unofficial/awesome-rust#applications

People are using rust as they do other general purpose programming languages that have somewhat of a focus on systems programming: to write code. Your question isn't really easy to answer in the same way that someone asking "what are people building with python?" isn't useful. The answer is everything from webpages to text editors to operating systems, and everything in between.