Cool webpage on days which have had violent tornadoes. You can graphically look at the watches, convective outlooks, and numerous observations and environmental analyses on the days of significant tornado events. It's a little tricky to play around with and get what you want on the day that you want it, but still great for looking at historical tornado days.
http://www.spc.noaa.gov/exper/outbreaks/#
http://www.spc.noaa.gov/exper/outbreaks/#