The National Zoo in Washington, D.C. is only one of several excellent zoos to be found in America, but it does have one compelling advantage over its counterparts: as part of the Smithsonian Institution, it's free.
While visiting this past weekend, I saw elephants, otters, a crocodile, several snakes--including one who was just a passerby, wandering along the sidewalk in a manner only slightly different from the rest of us--and a particularly endearing sloth bear. The giant panda exhibit no longer requires tickets, either, and it was fun to get to see Tai Shan, the adolescent panda born in 2005 to still-present parents Tian Tian and Mei Xiang on loan from China.
The lack of an admission fee has a surprising impact on the overall zoo experience. Since there's no need for visitors to pass through a central ticketing location, it's possible to enter the zoo from several different locations. The main entrance, for instance, is just a few blocks north of the Woodley Park/Adams Morgan stop on Metro (the Red Line), but those who prefer to drive will appreciate being able to walk directly into the park from one of five different lots.
Next time you make it out to D.C., plan to come by and see the zoo. You can't beat the price.