CSSBattle - Play against others in golf with your CSS skills
Ubersuggest - Want more traffic? Ubersuggest shows you how to win the game of SEO. Just type in a domain or a keyword to get started.
Indie Maker The Game - Open source game about indie makers
Keyword Explorer by Moz - Find the best keywords to improve your website's SEO traffic
Gamefroot - Gamefroot is the fun way to make games, animations, stories and share them with the world.
Content Marketing Stack - A curated directory of content marketing resources