You can even embed it in other things, like Wikis. https://www.mediawiki.org/wiki/Extension:GraphViz
Sometimes Excel when diagram could be made by drawing borders around cells and resizing rows and columns as per need. This helps when unplanned non-linear horizontal and vertical scaling is needed while making the diagram.
The SmartArt concept they introduced was quite promising, though I find the current state of it lagging. I am sad that it never picked up.
The downside of course is that it is not free.
I would also like to know open source alternatives to MS Visio. I can see that Ubuntu 12.04 comes with Libre Draw, but I haven't tried it yet.
Otherwise there is GIMP too.
+1 to all the other open source solutions mentioned here.