Just another get to know each other type problem because I think they're fun and I enjoy hearing what other people have built in R.
This could be a personal project, something for work or school, anything that you have enjoyed using R for.
Personally, I think that my favorite project that I've done is an ELO Model, similar to those at FiveThirtyEight, for the NHL. It started out as a personal project because I wanted to practice my R skills and thought it was interesting and then ended up using it for 2 different projects during graduate school. It isn't the best predictive model, only about 60% accurate, and I haven't figured out how to simulate the playoffs yet, but it's fun to play with and I've learned a lot about both R and the NHL.