Trivial Sierpinski triangle fractal implementation, drawn into a HTML canvas. Based on the "Chaos game" algorithm - drawing pointer moving towards randomly chosen triangle vertices.
See the 140byt.es site for a showcase of entries (built itself using 140-byte entries!), and follow @140bytes on Twitter.
To learn about byte-saving hacks for your own code, or to contribute what you've learned, head to the wiki.
140byt.es is brought to you by Jed Schmidt, with help from Alex Kloss. It was inspired by work from Thomas Fuchs and Dustin Diaz.
9 bytes smaller:
function(c,s,i,a,b,r){for(a=b=0;i--;r=Math.random())c.fillRect(a=(r>2/3&&s)+a>>1,b=(r<1/3&&s)+b>>1,1,1)}