Hackster.io is a community-driven center for all kinds of user-built projects. Lots of great creativity here!
Built by a team at Microsoft, this full curriculum takes you from farm to table with IoT. Highly recommended!
Go to Adafruit when you need to buy the bits and pieces you need to complete a project. It's the most extensive IoT store on the internet, great video content too!
SparkFun is a go-to resource to learn about hardware projects and buy the materials you need to make them.
The official Arduino development environment.