Next to Lua editing, Estrela's main use is for shader authoring. One can give shaders a quick test-compile through the use of NVIDIA's Cg compiler or DirectX? SDK (and in latest version there is also a simple OpenCL? offline compiler added).
Especially the Cg-based backend has numerous additions to aid shader authoring and analysis. The Cg compiler allows both Cg and native GLSL compilation.
While the screenshots show mostly Cg in action, the features apply to GLSL shaders as well.