yeah, while you're at it check out inigo quilez's website, as he's (i think?) one of the founders of shadertoy. he's a graphics master. his website holds a ton of incredibly useful and interesting graphics techniques, a bunch of raymarching stuff, procedural generation techniques, etc. an absolute goldmine if you're into graphics, graphics-related math, demoscene, or anything of that ilk.

http://www.iquilezles.org/

http://shaderfrog.com/ is cool too, neat to visualize it right in the browser

I'm a huge fan of http://shdr.bkcore.com since it lets me try out shaders in the same context I'm probably going to use them (on models as materials) with good old Suzanne, the blender monkey head.