At the Moose Family Center 926 Wayne Ave Silver Spring, until 7 pm. Go up Fenton, turn left on Wayne, park at the County parking garage that is on your immediate right, and the Moose Family Center is right across the street. You will get soft drinks, pretzels and a great MASH-looking T-shirt.
Blood supplies are always disastrously low around late August and Labor Day. If you can't get to the downtown Silver Spring location today, check the Red Cross donor information pages for the next location near to Takoma Park. You can also make an appointment online.
Although only 5% of the population donates blood, the chance is 97% that at some point someone you know will need blood. More statistics.
Are you Jewish? Many believe that if one is able to give blood, doing so is obligatory, not optional. It is an act of Pikuach Nefesh, "Saving a Life." The Islamic position is similar. (See this as well) From the Quran: To save one life is to save all humanity.
Statements from various religious representatives concerning tissue donation.
The Texas list.
... and another version.