Google webmasters forum is the best place to view penalised websites, and what they did. Every 5 minutes somebody posts the now familiar cry of, "yesterday i was no 1 for 5 search results, today im no 374!" , after the initial sob story, the truth is usually found to be some dodgy link scheme, link farm, paid links, google bowling, or just plain bad SEO. In my experience, there appears to be endless possibilities to gain a penalty.