Do I need an Artificial Intelligence API?
One needs to use an artificial intelligence (AI) API, if there is a need to add AI functionality to a software application - this is pretty obvious. Traditionally, my advice on machine learning (ML) software includes the following two excellent curated lists of resources: this one and this one.

However, keep in mind that ML is just a subset of AI domain, so if your tasks involve AI areas beyond ML, you need more AI-focused tools or platforms. For example, you can take a look at ai-one's AI platforms and APIs as well as interesting general AI open source project OpenCog.

In addition to the above-mentioned AI-focused platforms, IBM's Watson AI system deserves a separate mention, as quite cool and promising. It offers its own ecosystem for developers, called IBM Watson Developer Cloud, based on IBM's BlueMix cloud computing platform-as-a-service (PaaS). However, at the present time, I find this offering to be quite expensive as well as limiting, especially for individual developers, small startups and other small businesses, due to its tight integration with and reliance only on a single PaaS (Blue Mix). It will be interesting to watch this space as competition in AI domain and marketplace IMHO will surely intensify in the future.