As with Fragmentarium, in Synthclipse shaders tweaking is especially easy: using simple annotations in a GLSL code comments uniform variables can be bound to a Graphical User Interface elements like sliders, buttons or check boxes.
Synthclipse is based on Eclipse IDE and depends on Eclipse CDT (C/C++ Development Tooling). It pretends that GLSL is just plain C++ code. The reason why GLSL is interpreted in Synthclipse as C++ is to get a great editor with a lot of functionalities, for free. To name a few: auto-complementation, function and globals list (Code Overview), simple static code analysis, a lot of key shortcuts, configurable syntax coloring, code autoformatting, advanced search/replace, "go to function/header by CTRL + click", etc. It is not a perfect solution, but it works surprisingly well most of the time.
As in Fragmentarium, Synthclipse extends GLSL language by "#include" preprocesor directive. It works exactly as in C/C++ and it allows shaders to be more modular. It is the only GLSL extension, as opposite to Fragmentarium. All other "magic" happens in special comments - called Commands - started by "//!" or "/*!".
You can find out more about Synthclipse features and syntax in the User Guide.
Added Update Site for Eclipse: http://ringojingosden.com/sclipse/update/ (Thanks to Dimitri Velovski for hosting).
Now one can install Synthclipse from an existing Eclipse instance.