Flight is incredibly extensible. There are a number of plugins that can be used to add functionality to your Flight application. Some are officially supported by the Flight Team and others are micro/lite libraries to help you get started.
Caching is a great way to speed up your application. There are a number of caching libraries that can be used with Flight.
Cookies are a great way to store small bits of data on the client side. They can be used to store user preferences, application settings, and more.
Debugging is crucial when you are developing in your local environment. There are a few plugins that can elevate your debugging experience.
Databases are the core to most applications. This is how you store and retrieve data. Some database libraries are simply wrappers to write queries and some are full fledged ORMs.
Encryption is crucial for any application that stores sensitive data. Encrypting and decrypting the data isn't terribly hard, but properly storing the encryption key can be difficult. The most important thing is to never store your encryption key in a public directory or to commit it to your code repository.
Sessions aren't really useful for API's but for building out a web application, sessions can be crucial for maintaining state and login information.
Templating is core to any web application with a UI. There are a number of templating engines that can be used with Flight.
Got a plugin you'd like to share? Submit a pull request to add it to the list!