If you're looking for a site to make predictions about real-world events and compete against others to win prizes, Metaculus is a great option. This community-driven site covers topics like Biosecurity, AI Progress, Nuclear Security, and Climate Change. It offers a forecasting interface and Forecasting Tournaments, where you can submit predictions and compete for prizes. Metaculus is designed to be community focused, transparent and publicly accountable, and it can be a useful tool for individuals and organizations trying to address global challenges.
Another interesting site is AIcrowd. Although it's geared more toward artificial intelligence challenges, it offers a variety of opportunities for data scientists and enthusiasts to tackle real-world problems. AIcrowd challenges cover areas like procedural generation, image-based food recognition, and natural conversations, with prizes and other incentives. The site encourages community involvement, so you can look at winning solutions, discuss challenges, and find collaborators for research projects.