Draw.io is free but requires you to do so much work yourself to create objects, size them, group them, connect them, arrange them.
Most other tools I've found when googling around require the same level of effort to use as Draw.io: AsciiFlow, Gliffy.
Omnigraffle is out of the picture since I need to be able to share and use graphs across Mac, Windows, and Linux users -- without needing to describe multiple tools and conversion between formats or anything.
The simplest tool I've seen that just works is Visio. It is primarily click-and-drag and has great suggested connecting, sizing, grouping, and arranging features. It has an online client that works in Chrome in Mac, Linux, Windows, and FreeBSD. On Windows there is also the fully-featured desktop client. It's also familiar/intuitive enough to old-school Office users and feels like a regular graphics application to those of us less Office-oriented.
I don't enjoy spending as much time as I have hunting for the ideal solution. But I'm happy to have found Visio.