Figma is very effective for UI Design projects, especially when it comes to working on teams. Their live collaboration and version control feature abilities make it my go-to tool when collaborating with others. It's also free and easy to pick up, making it available to everyone.
Dribbble - Discover the World's Top Designers & Creative Professionals
Dribbble is an online community for showcasing user-made artwork. It functions as a self-promotion and networking platform for creatives in areas such as graphic design, web design, illustration, and photography. It is a great tool to learn more about how members push their creativity and gain inspiration from others.
Although this resource was made for hackathon events, the learning guides and kits cover many key topics important for any UI related project.