I challenged myself to write a tweetable one-line Mandelbrot set, and this is the result. 135 characters long, generates an ascii art Mandelbrot.

135CharacterMandelbrot.js

o='',s=0.02;for(l=-1;(l+=s)<1;){for(k=-2;(k+=s)<1;){for(x=y=0,j=99;x*x+y*y<4&&--j;){a=x*x-y*y+k,y=2*x*y+l,x=a}o+=j?'#':'_'}o+='<br>'};
document.write(o);document.getElementsByTagName('html')[0].style.lineHeight = 0.5;

Ok, I cheated a little by breaking out the code that outputs the set to the browser and formats it a little better into a second line. Any suggestions on how to fold that into the 140 character limit would be much appreciated.

Copy and paste the gist into any random browser js console.