How to generate schemas of GIS hardware / software / workflows?

yEd is another great free diagram editor you can use. It is similar to Visio, and quite easy to use.

I'd have to guess that the bottom one was done in some form of Microsoft Visio - which of course is not free. Here are some of those shapes from Visio 2010 that I just found on my install, and they look very similar to the graphic you show:

I have used InkScape for making all sorts of technical diagrams, posters, and flyers. It's completely FOSS, and if you have any experience with Adobe Illustrator, the learning curve isn't too bad. I often will Google for images I want to place into my diagram or there are tons of sites out there with SVG graphics you can use.

Another option, which I haven't tried, could be OpenOffice Draw. It is also FOSS and appears to have connectors like Visio and you can import common graphics formats.

As far as graphics you can find, Smashing Magazine has awesome posts on free icons sets that are out there (search the site for "icons" for more). I have found some real gems there, such as nice globes, computers, etc. Some of the icon sets even provide the vector formats of the icons, which is nice. These icons work great in pieces like what you are wanting to create.

I guess what I'm getting at is that, with a little creativity and Googling, you can easily make professional-looking diagrams with open-source drawing programs.

Take a look at the open-source project called Dia. Some older articles here and here describe its functionality and give it pretty good review as an alternative to Visio.

I have also used the Pencil Project software, in which you can quickly prototype GUI's or workflow diagrams in a "sketch" fashion (as if you were drawing on the back of a napkin, clearly meant to be a rough sketch, not a finalized design).

Some quick googling found some compiled lists (below) which compare many alternatives and give a quick introduction to each:

• 7 Free Diagramming Tools To Create Diagrams Easily – Alternative to Microsoft Visio
• Open source Visio alternatives
• Free alternatives to Microsoft Visio