I came across this site recently, thanks to Carl from .NET Rocks

It's a free online tool for creating flow charts, process diagrams, mindmaps, wireframes, the whole lot.

It's called draw.io. You have multiple options for where you can save your document including Google Drive, Dropbox, OneDrive or your computer.

It's really easy to use and it doesn't feel like it's in a browser when you are using it. I'm really impressed with it. It's a pleasure to use.

I've not compared the full list of features with Visio, but as far as I can tell you get all of the features you need.

Have a play and make sure you share this with your friends, they are going to thank you for it.

https://draw.io/