Thanks to The Freebie Blogger for this info:
It looks like Smithsonian Magazine will be offering free museum admission again this year. This will take place on 9/26/09 at museums across the country. You can click here to find a list of participating venues (there may be new locations added later). The admission card page is not up yet, but when it is you can go there to print out your free admission card. Card is good for you and a guest, limit one per household.