Founders Nation - Find or become a startup cofounder
CoFoundersLab - Meet 250,000+ entrepreneurs online
Co-founder Question Cards - Questions to ask your co-founder to know each other better.
Findnlink - Find people to work with on your ideas.
Maker Network - Explore the Product Hunt Maker network
Y Combinator Co-founder Matching - YC’s free online platform to help you find your co-founder