Check out Programmable Web for the most popular API directory. Here you can browse and compare APIs, catch up on news, and learn more about API technology.
If you have a specific use case in mind you want to solve with an API, comparing APIs on a directory could help you find your answer! If you're an API provider, directories can aid discovery of your API, and the site metadata can help you find your market.
An API directory is just a list of APIs. It allows developers to discover and compare APIs, but not to use and experience them the way an API marketplace does. It doesn't offer authentication functionality, ecommerce, or access tokens, just API discovery.
An API directory is a good way to find and compare APIs, but doesn't allow you to try them out the way an API marketplace does.