CNN's fear and greed index in REST API form, courtest of RapidAPI. It's a free API.

Seemingly inspired by CNN's index of the same name, this site crunches and calculates cryptocurrency-related sentiment and comes up with a numerical index of what appears to be influencing the markets at that time, vis a vis, fear or greed. Has a free to use REST API.

Analyze a company's investment profile by looking at pertinent news, stock price patterns, and overall sentiment.

Has a REST API. Free tier offers 1 API key, 60 requests per minute, 1000 requests per day. The premium tier isn't too bad ($25us/month) if you need to heavily use their service.

A bot implemented as a Github App which analyzes the interactions a user has had elsewhere on Github and uses sentiment analysis to figure out how toxic the user is likely to be in their interactions with your project.

Uses the Probot framework.

VADER (Valence Aware Dictionary and sEntiment Reasoner) is a lexicon and rule-based sentiment analysis tool that is specifically attuned to sentiments expressed in social media. It is fully open-sourced under the MIT License]. Incorporated into NLTK.

Online service for doing textual analysis of documents (sent to and online). Realtime analysis. Offers SDKs, so the REST API is probably in there somewhere. Sentiment analysis (general and domain-specific). Classification and categorization. Entity and concept detection and extraction. Hashtag suggestion.

3 hits/second
1000 hits/day for free

By looking around in here you can tell what the API rails are and how to use them:
https://github.com/AYLIEN/aylien_textapi_python/blob/master/aylienapiclient/textapi.py#L84

extract, classify, concepts, entities, hashtags, language, related, sentiment, summarize, microformats, image-tags, classify/unsupervised (picks the most semantically relevant class label or tag), combined (runs multiple analysis operations in one API call).

headers = {
    'Accept': 'application/json',
    'Content-type': 'application/x-www-form-urlencoded',
    'X-AYLIEN-TextAPI-Application-ID': application ID,
    'X-AYLIEN-TextAPI-Application-Key': application key,
    'User-Agent': "Aylien Text API Python " + aylienapiclient.__version__ (0.6.0 as of right now)
    }