I don't do a lot of diagramming...But I've found Yed[1] to be nice. Free & Cross Platform. [1] https://www.yworks.com/products/yed https://www.yworks.com/prod

I don't do a lot of diagramming...But I've found Yed[1] to be nice. Free & Cross Platform. [1] https://www.yworks.com/products/yed https://www.yworks.com/products/yed