HackADay is described as 'Hackaday.io is a platform for people who like to build things' and is an website in the Social & Communications category. There are five alternatives to HackADay, not only websites but also apps for a variety of platforms, including Android, Windows Phone, Self-Hosted solutions and SaaS. The best alternative is Instructables. It's not free, so if you're looking for a free alternative, you could try Wikifab or Hackster. Other great sites and apps similar to HackADay are Wikifactory (Free Personal) and Piblitz (Free).
Wikifactory is a social platform for collaborative product development. It offers an all-in-one workspace to accelerate the way your team works. Inspired by best practices in software development, built for product developers from the ground up.