Experimental algorithm for gradient noise.
You can find the formula and a detailed explanation here.
See it in action here.
Please note the underlying PRNG is questionable.
A bit more eye candy here. Go to shadertoy.com and copy/paste this script into a fresh shader.
I would love to share the shader myself on shadertoy.com, but I'm having trouble verifying my email address there. Sorry.
Build and run noise2D.c, redirecting its output to a file with extension .pgm. Open the file with an application that accepts raw PGM format, for example GIMP.