Here are the data-related API's I wish existed:

1. Indeed.com - analytics on how many job ads mentioned a specific brand or keyword.

2. Yelp - analytics on how many checkins a restaurant or chain got every month.

3. Apple Store - analytics on how many downloads an app got every month.

4. Amazon Reviews - an API to retrieve reviews for a product.

5. Google Trends - API to retrieve historical trends for a keyword

6. Google Search API - API to get search results for a keyword

7. Linkedin Company Page API - API to get the feed for any company page

8. Instagram API - API to get the feed for any instagram user or search results for any keyword

Here are the non-data API's I wished exist (that could be potential low-hanging fruit startup ideas):

....None