First, I have to say that I am a little torn. It is nice that people are offering discounts for Memorial Day, but shopping isn't really the point of the holiday. Regardless,
Kings Dominion Amusement Park (outside of Richmond, Virginia) is offering free admission for active duty, retired and former military members on Sunday, May 30th and Monday, May 31st. A current military ID or discharge papers are required to obtain the ticket. In addition, the same people may purchase tickets for up to six members of their immediate family for the discounted rate of $33.99 each. They recommend that tickets be purchased at ITT and MWR locations to avoid long lines at the park.
Borders Bookstore is offering 10% off purchases Friday, May 28 through Monday, May 31st. The discount is available to active duty members and families. A valid military ID is required. I couldn't find a link to an official announcement so I'd call before you travel any distance to a Borders.
There's a great state-by-state list available at Hot Coupon World.
Please add other deals in the comments. Thanks!