As someone who has been trained as an educator and not a programmer, I have largely been on my own when it comes to learning computer science so that I can teach it to students. My students start working with Scratch (MIT) and some move on to do robotics with Modkit and RobotC, however I wanted to find a FOSS alternative that is more general purpose than the introductory applications, and could also be used with Raspberry Pi. Out of the various languages I have looked at, Python is the only one which has proven itself to be general purpose, powerful, user-friendly, and easy enough for younger (elementary) students to grasp.
I was able to learn the basics easily on my own, and it's not at all difficult for someone proficient in lesson planning to begin integrating Python into projects. Its extensive library allows for plenty of scaffolding in lessons and helps to avoid frustration which stems from minor errors. I later found that it can even be used with prototyping tools such as Arduino and the Omega.
At first, GIMP doesn't seem all that user-friendly - especially to someone who is unfamiliar with image manipulation software in general. However, there are a number of tutorials available (which are free and pretty easy to find), and even a compete novice can get the basics down pretty easily. As an educator I have attempted to introduce students to this software, and have found that for middle school students it can be used relatively easily with some well-structured guidance and some templates. I have found that students in seventh grade and higher catch on quickly after being introduced to the application and many find it very enjoyable. GIMP is undoubtedly a powerful piece of software that is very useful in K-12 schools, and there's certainly nothing out there that can beat the price.
This is a piece of software that someone would expect to pay a significant amount of money for (the Microsoft equivalent is priced at something like $140). In addition to the main application, there are a significant number of add-ons available including different aircraft and scenery. The degree of customization this software allows the user goes beyond anything I have seen in any other flight simulator. There seems to be a very dedicated group of developers behind this project because it was very easy to find documentation, tutorials, and other resources.
I initially tried FlightGear out on my laptop while previewing different applications, and quickly discovered that it's a hog when it comes to system resources. Despite not having a laptop made for games of high-intensity graphics, however, the program still ran pretty smoothly. Playing with just the keyboard quickly proved to a pain, and I found that hooking up my relatively inexpensive ThrustMaster (X-Flight) joystick provided a much more enjoyable experience. I'm not sure if FlightGear is everything that X-Plane is, although I haven't messed around with it enough yet to tell; so far the two seem to have different strengths and weaknesses. For a free piece of software, however, this is quite impressive!