The Instapundit recommends a wet/dry vac for every home in light of his recent plumbing problems.
I have one, bought after my washing machine broke and flooded large sections of my finished basement. The problem with a wet/dry vac's usefulness in plumbing emergencies is that you must know an emergency exists. The second use for my wet/dry vac came after a valve in my toilet failed (the one that shuts off water flow when the tank fills). However, we were not at home when it happened. Fortuneately we came home before the entire middle level of our townhome was destroyed.
Still, having it and being able to clean up before the insurance provided clean up crew comes to the house was a big help in alleviating further damage.