The Night of Museums.

For one night only, 1700+ museums and attractions in France will be participating in "The Night of Museums" which means they will be open well into the night (long past usual closing hours) and offering admissions for free. This event is the second edition, and will take place in several European countries. If you happen to be in France on May 20, 2006, don't miss this wonderful cultural opportunity! Plan your days for other sights, but spend your evenings visiting multiple museums at no charge. This is a guaranteed winner.

What makes The Night of Museums so unique and exciting, aside from the free admission, is the fact that many art pieces and architectural structures appear surprisingly different under artificial lighting. Some attractions are taking advantage of this, using multi-colored lights to put on full shows.  Learning while having fun. Brilliant!

Since you probably won't have the time to see all of the free events, don't be shy about asking people on the street (or here as a comment) which ones might be best. They may offer recommendations on what to see in which priority.  Also, here is a map of France. Simply click on your region and then select your city to see the participating museums and attractions. Take note of the hours and schedule your itinerary:
Night of Museums events

Have fun!