If you want a diagram you can embed in text and check into source control, take a look at GraphViz. http://www.graphviz.org/

You can even embed it in other things, like Wikis. https://www.mediawiki.org/wiki/Extension:GraphViz

Microsoft Word and PowerPoint.

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.

I had started with using MS Visio at my job. Though I had absolutely no experience in making diagrams, Visio was quite easy to get started and become comfortable in quick time.

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.

LibreOffice Draw is reasonable if you are coming from Linux.

Otherwise there is GIMP too.

+1 to all the other open source solutions mentioned here.

I use Dia: https://wiki.gnome.org/Apps/Dia

Yeah I quite like Gliffy. More often than not, we take photos of whiteboards and email them round the team.

Give www.draw.io a go. I have used it a few times at work.

Gliffy(www.gliffy.com) works great for me.

Blockdiag

Balsamiq

Inkscape