This is an open-source QuickLook plugin to generate thumbnails and previews for WebP images. Requires Mac OS X 10.7 or later (but tested on 10.8).
New version supports WebP lossy, lossless, and alpha channels.
Click Releases and download the latest version. Unzip it, and put WebP.qlgenerator into ~/Library/QuickLook (or /Library/QuickLook). You may need to reboot to enable it (or open Terminal and run qlmanage -r).
The plugin uses the precompiled libwebp.a (i386, x86_64; git rev. 2afee60a7) located in lib. I build it manually, and haven't yet created a script to build it from sources; patches welcome.