CoFoundersLab - Meet 250,000+ entrepreneurs online
Founders Nation - Find or become a startup cofounder
Co-founder Question Cards - Questions to ask your co-founder to know each other better.
CollabFinder - Find cofounders and makers to help build your project
Team of Teams - http://teamofteams.io/ digital platform helps young co-founders to match by their professional core values with help of ML.
Findnlink - Find people to work with on your ideas.