Founders Nation - Find or become a startup cofounder
Startup School - How to start a startup, by Y Combinator
Find A Maker - find a partner for your next project
Co-founder's Daily Journal - One of the key issues founders are struggling with is co-founder conflict.
CoFoundersLab - Meet 250,000+ entrepreneurs online
Co-founder Question Cards - Questions to ask your co-founder to know each other better.