It seems that this format is a Google proprietary format released under BSD license.
It has been developed, based on a video encoder format VP8, with the scope to reduce image size for the rilief of Google storage space and network traffic speedup.
It includes a loss and lossless algorithm, but the image quality is poor compared to native JPG or PNG, and limited in dimensions. Currently is under development a new version that is supposed to overcome the quality problem, but it appeared to not be an easy task and for this reason Google relocated the WebP2 project to a research environment:
"playground for image compression experiments".
The repository where are available sources and binaries is that reported by MrBCX.
Actually the format is handled by chromium browser and its derivatives, including MS Edge.
The format isn't supported in GDI+, but Google implemented a plugin for WIC (Windows Imaging Component) that integrated the format in the applications that use WIC as Windows Photo Viewer, Microsoft Office 2010, FastPictureViewer.
Conclusion: this format is very useful for web pages to speed-up loading and reduce storage that will make it largely diffused in web apps, but is resource expansive (the encoding/decoding is CPU intensive), and poor in quality (actual standard). The last is a limit for high quality pictures.
Last, but not least, if a giant as Google push it, be sure that it will be a de-facto standard shortly....
For more info
see.