As a starting point...first read about the built-in apps. (simple lighting, Rule Machine, notifications, Amazon Echo, etc)
Then, install the app linked below. Someone @dman2306 wrote a package manager and a lot of the most popular and more mature apps/drivers have provided manifests. Once you install it you can browse that library. Definitely a good place to start.
Just realize that some of the apps provide functionality you could do yourself in "Rule Machine" if you preferred, but they might just be big time savers.
After that look in the Developers category on these forums for more and more niche stuff.
