Kirl: Well, DisplayHack is easy for me to whore since I'm an author there, hehe. I don't know of many sites quite like it other than Flipcode, but that's a bit dated..
Raizor: Yeah, it's step size. In raymarching there can be quite a big difference in step size between pixels (and different parts of the ray, for that matter) so the uneven-ness produce artifacts.