Founders Nation - Find or become a startup cofounder
Find A Maker - find a partner for your next project
Findnlink - Find people to work with on your ideas.
CoFoundersLab - Meet 250,000+ entrepreneurs online
Y Combinator Co-founder Matching - YC’s free online platform to help you find your co-founder
Founder's Prenup - Dating Co-Founders? Secure what's yours!