Rocketgraph - Rocketgraph is an open-source drop in replacement to Firebase, except it's much better.
Steam Charts - An ongoing analysis of Steam's concurrent players.
Discover on Demand - Just like Spotify's Discover Weekly, but whenever you want.
Steam Database - This tool was made to give better insight into the applications that Steam has in its absolutely huge database.
Plug - Lightweight OSX player for Hype Machine
Steam Inventory Helper - Steam Inventory Helper is a free browser extension for Google Chrome that adds a big amount of new features into the Steam trade-offer system & Steam store functionality.