It used to be that Instalanches were the gold standard for blowing out your server bandwidth. An Instalanche occurred when Glenn Reynolds (or one of the other super-large bloggers) pointed his enormous traffic to an article of interest in some small blog. The results were sortof like hooking your transistor radio to a high-tension power line. This was the traffic chart from my first instalanche.
Today, most of my huge traffic spikes come when an article of mine moves up high on reddit or stumbleupon or del.icio.us (I don't remember ever getting much from digg). Right now my article on water pricing is on the front page of reddit, and the traffic spike is enormous.