I love to show off other website's great features...and Wunderground (one of the best weather websites out there) has a new feature that's pretty darn awesome. Their wundermap feature has been around for some time...showcasing radar, weather observations in real time, but also including model data. It now includes past weather as well.
This feature has been there for at least a little while -- I'm not sure of the exact date they started running this feature but I was made aware of it this morning while browsing around the site. Radar data appears to go back into the mid 1990's...they have the January '96 snowstorm (below) within their radar image archives and I was able to look at some shots of Hurricane Isabel (above) as well.
In addition to radar loops, they have temperature observations as well on the page so you can get a sense of how cold/warm/in between a given day was going back at least into the late 1980's using the map function (I was able to go back to January 1988 in a quick browse of the site and obtain data)...or before that if you looked at a climate site for a given day. When you click the link, the time function on top of the map will allow you to travel back in weather time if you so desire.
In a world full of great places that will suck you in and suck your time away, this site might be the weather geek's Angry Birds and solitaire rolled into one convenient spot.