What would be a good (free) Ada IDE for either Linux or Windows?
It's not an IDE per-se, but I use SciTE for Ada code editing.
Since I also use it for pretty much everything else, I've got it configured to be convenient. SciTE is configurable and has a lot of interesting scripting options to bring it up on-par with full blown IDEs. And, it's very lightweight (fits a floppy) and open source.
I had worked with GNAT previously and found it quite well. There is a free version that can be found here: https://libre.adacore.com/
GNAT Programming Studio:
https://libre.adacore.com/gps/main.html https://libre.adacore.com/index.html
AdaGIDE (windows only):
http://sourceforge.net/projects/adagide/
SciTE does not support code folding, but does a good job with Ada.
I like the Emacs Ada mode (http://www.stephe-leake.org/emacs/ada-mode/emacs-ada-mode.html). The only downside is that you have to learn Emacs, but really, isn't that actually a plus?
https://libre.adacore.com/GNATbench/
GNATbench - Ada plug-in for Eclipse is included in GNAT GPL Edition
For Windows, there is also GNAVI:
GNAVI is the open source alternative to visual software development languages like Delphi and Visual Basic. In addition to just being fully Open Source under the GPL, the language foundation of GNAVI, unlike Pascal or Basic of its competition, is the international standard of engineering, Ada. GNAVI for Windows offers comparable features to Delphi and Visual Basic including use of Active X controls and the ability to interface with .NET and Java.