I love doing stuff like this for fun. Sometimes I try to get nice algorithms into tweets. Here is one of my favorites so far, returns the powerset for array 'a':

            function P(a){for(p=[],i=Math.pow(2,l=a.length);i;)for(b=i.toString(2),j=k=b.length,p[--i]=[];j;)if(b[--j]==1)p[i].push(a[j+l-k]);return p}

I know that there are some big gotchas here, such as b and p being declared globally due to lack of var...but I gave up trying to gain an extra 4 bytes for function(a,b,p). Also the subset arrays are in reverse order.