We at Visual Perspective just released an open source, web-based, remote sensing viewer (https://iceland.visualperspective.io/). With this viewer, you can explore Iceland’s vegetation over space and time. There is an accompanying blog post which goes over Normalized Difference Vegetation Index (NDVI) and how to use the viewer (https://visualperspective.io/blog/iceland-ndvi-viewer).
Source code is at: https://github.com/VisualPerspective/ndvi-viewer WebGL is used for the display, but also for GPGPU averaging of NDVI in the selected region.