Founders Nation - Find or become a startup cofounder
CoFoundersLab - Meet 250,000+ entrepreneurs online
Y Combinator Co-founder Matching - YC’s free online platform to help you find your co-founder
CollabFinder - Find cofounders and makers to help build your project
Nugget - Find your perfect startup idea
Co-founder Question Cards - Questions to ask your co-founder to know each other better.