Today relying on a GPU solution translates to vendor lock-in, it makes you much more dependent on a particular set of hardware and even a particular OS/driver stack.
This could be very useful for running certain algorithms in web apps...
Below I've linked github urls and two blog posts that were done about them. Unfortunately right now webGL is kind of a moving target and I doubt these examples still work out of the box. However, if you are interested in GPGPU on webGL I would encourage you to get involved as I think there are some tweaks to the standard that would make GPGPU life easier.
Matrix Mutliply http://learningwebgl.com/blog/?p=1828 http://github.com/bunions1/matrixMultiplyGpu
Falling Sand http://learningwebgl.com/blog/?p=1471 http://github.com/bunions1/fallingsand-webgl