Dave Baum
Native byte-code compiler.
A C-like language, called (of course) Not Quite C. Should be pretty easy to learn for anyone with even minimal coding experience.
GNU/Linux, MS Windows, and Macintosh.
NQC is a byte-code compiler that takes programs written in a C-like syntax and compiles it (on the PC) into byte-code that can be understood by the standard Lego firmware. This approach has strengths and drawbacks: for example, the standard firmware can handle only 32 variables and so NQC is similarly limited. However, you can do a surprising amount within these limitations. Setup is pretty simple and the project as a whole is very well documented. This is also probably the most popular alternative programming system, so there are a lot of people willing and ready to help out if you start using NQC.
As already mentioned, NQC is the only option (right now) that supports the Cybermaster and Scout products under Linux. Furthermore, a beta version of it works with version 2.0 of the firmware, making it the first alternative programming system to support the added functionality of the new firmware.