Founders Nation - Find or become a startup cofounder
Peanut - Peanut is a Mobile Application for moms developed and published by Peanut App Ltd.
CoFoundersLab - Meet 250,000+ entrepreneurs online
CollabFinder - Find cofounders and makers to help build your project
PenPal World - Connect with people all over the world
Co-founder Question Cards - Questions to ask your co-founder to know each other better.