Mermaid is the fastest tool to write diagrams after pen and paper. I use it with Markdown Enhanced, it's a powerful tool that quickly ensures your entire team is on the same page and understand the ideas/concepts you're discussing. Started adding exported images in confluence with the source code attached to the same page, people always get amazed and started adopting it (slowly, I'd say). The speed is great but not being attached to a heavy tool such as Visio or Gliffy is the game changer.