Startup School - How to start a startup, by Y Combinator
Founders Nation - Find or become a startup cofounder
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.
CollabFinder - Find cofounders and makers to help build your project