Shdr is described as 'online ESSL (GLSL) shader editor, viewer and validator powered by WebGL'. There are more than 10 alternatives to Shdr for a variety of platforms, including Online / Web-based, Windows, Linux, iPhone and Self-Hosted solutions. The best alternative is Shadertoy, which is free. Other great apps like Shdr are Shader Editor (Free, Open Source), SHADERed (Free, Open Source), Shade - Pro Shader Editor (Freemium) and GSN Composer (Free).
KodeLife is a fast, lightweight, live-coding IDE for shader development. It's available natively across Mac, Windows and Linux desktops and supports all flavors of OpenGL GLSL, as well as platform specific shading languages such as the Metal Shading Language and DirectX HLSL.