Smithsonian Magazine will be offering free museum admission again this year. This will take place on 9/26/09 at museums across the country. 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.
To find participating venues, go here: Museum Day 2009 | Smithsonian.com
Here is the admission card page:
Museum Day 2009 | Smithsonian.com