I just discovered this one last week while checking www.dealnews.com and it's a goodie!
Smithsonian Magazine is having its 6th Annual Museum Day on September 25, 2010. Visit the website to print out a ticket for two free passes to a participating museum (and there are many participating!). It's a steal, for sure.
As for me, I will be visiting The Franklin Institute in Philadelphia. Adult tickets are typically around $15 per person. That's $30 savings for the hubby and me! Wahoo!