Steem Developer Portal
Radiator - https://github.com/inertia186/radiator
Radiator is a Ruby API client to interact with the steem blockchain.
SteemJS - https://github.com/svk31/steemjs-lib
A python library to interact with the STEEM blockchain. It includes the CLI tool beempy.
Piston-cli - https://github.com/xeroc/piston-cli
This python library is unmaintained! Swiss army knife for interacting with the STEEM blockchain.
ChainSync - https://github.com/aaroncox/chainsync
A simple library to stream blocks and operations for digesting into other mediums.
SteemIAPI - http://steem.esteem.ws/
Interactive Steem API swagger, open-source script allows you to interact with blockchain.
PHP Steem - https://github.com/lukestokes/php-steem-tools
Various tools and scripts written in PHP for exploring the STEEM blockchain.
An API Wrapper for Steem written in Java
GoSteem - https://github.com/go-steem/rpc
Golang RPC client library for Steem
SteemClientRS - https://github.com/cyberpunk-ventures/steem-client-rs
Client library for Steem blockchain built with Rust
Services and databases
SteemSQL - http://www.steemsql.com/
A private Microsoft SQL server database with Steem blockchain data, subscription based, allows you to do flexible queries and analyze blockchain data.
Steem Data - https://github.com/SteemData
SteemData is discontinued but open-source python project, it parses the STEEM blockchain for you, and provide the data as a fast and convenient MongoDB service. SteemData helps developers and researchers build better STEEM applications.
SteemData-Ruby - https://github.com/steem-third-party/steem_data
Wrapper for accessing steemdata.com in a ruby application. It uses MongoID and ActiveModel to leverage all of the tools provided by those libraries.
eSync extracts Steem blockchain data and saves into Mongodb, written in Nodejs.
Glasnost is an GraphQL app server for Steem blockchain built with Elixir, Phoenix and Mnesia.
Most people who will be implementing OAuth2 will want to find and utilize a library in the language of their choice. These libraries, while not necessarily built by Steemit, should support the SteemConnect API.
SteemConnect SDK - https://github.com/steemit/steemconnect-sdk
steem-connect-firebase-function - https://jakipatryk.github.io/steemconnect-firebase-functions/
A TypeScript library that can help you build applications with SteemConnect and Firebase.
Sc2 SDK PHP - https://github.com/hernandev/sc2-sdk-php
Easily integrate STEEM blockchain into your PHP applications, though SteemConnect.
Social Auth SteemConnect - https://pypi.python.org/pypi/social-auth-steemconnect/0.0.2
Pluggable authentication backend for python-social-auth, that allows authentication via SteemConnect (v2).
steemconnect-python-client - https://github.com/emre/steemconnect-python-client
steemconnect-python-client is a simple yet powerful library to interact with the Steemconnect.
omniauth-steemconnect - https://rubygems.org/gems/omniauth-steemconnect/versions/0.1.0
Ruby Omniauth2 Strategy for SteemConnect.
SwiftyConnect - https://github.com/caspernikus/SwiftyConnect
SteemConnect Library for iOS / Swift.
SteemConnect4j - https://github.com/hapramp/steemconnect4j
Steemconnect SDK for Java.