Steam API:
API Documentation (Unofficial): https://wiki.teamfortress.com/wiki/User:RJackson/StorefrontAPI
ALL: http://api.steampowered.com/ISteamApps/GetAppList/v0001/
Single Game API: https://store.steampowered.com/api/appdetails?appids=57690
Steam Spy:
API Documentation: https://steamspy.com/api.php
ALL: http://steamspy.com/api.php?request=all
Single Game API: steamspy.com/api.php?request=appdetails&appid=730