NOAA Weather Service API
Many people may want information pertaining to weather data. However, most weather APIs charge for their services. In contrast, since NOAA Weather is an official US-government service, their API is fully free-to-use (assuming you supply your email address in the header of the HTTP request, so they can contact you if you are using their API too much).
This handy API can be used to access numerous amounts of weather data; however, the feature that I find most-useful is searching by airport. For example, I can request the weather data for Austin Straubel or O'Hare Airport. It gives me the current temperature (in Celsius) as well as other related data about the weather at the airport. As long as the airport is fully-located in the United States (no support for international cities unfortunately), the API is fully-free to use.
Government COVID-19 Reporting API
Since COVID-19 reporting is traditionally done by the Centers for Disease Control (or CDC), you can get your information "right from the horse's mouth" (so-to-speak) by using an official government API. According to the documentation's website, the CDC reports data on COVID-19 cases and deaths and this data is regularly posted to the API. This data is available in many formats, including CSV, XML, and JSON.
Kanye REST
As a little departure from the more-useful (and sometimes serious) APIs, the Kanye REST API tries to make life more enjoyable by cracking periodic jokes whenever the API is accessed. In contrast to the other APIs mentioned above that are quite complex, the Kanye REST API is quite simple (containing only a quote). Since the API is designed to be quite simple, it is only available in the JSON format.
Summary
Although many APIs cost money to use (such as those created by private companies), some APIs have free tiers or are outright free to use. For example, the US Government contains a catalog of free APIs available on their website. These APIs are provided in many formats (such as XML or CSV); however, most web developers have gravitated towards using the JSON format as the new standard due to its tight integration with the JavaScript runtime. For those looking for simple government-provided data, free APIs are hard to beat.