Washington has a well-known height restriction on its buildings; you'll find no places with panoramic views of downtown or the monumental core within the District.
The rooftop scene is alive and well during the summer months, however. If you just want a view of the local neighborhood and perhaps the Washington Monument (which can be seen from a significant proportion of the area), there are dozens of restaurant, bar, and hotel rooftops in various neighborhoods: I have taken visitors for example to the Kennedy Center Roof Terrace with views of Georgetown and the Potomac; ADC at the Donovan House Hotel on Logan Circle, The Observatory at the Graham Hotel in Georgetown, and The Reef in Adams Morgan.
Probably the most popular venue-with-a-view is the unimaginatively named P.O.V. Roof Terrace atop the W Hotel at 15th and F St. NW near the White House. Visible from the windows are the section of the National Mall near Washington Monument, the White House, EEOB, and the Treasury Building. Long waits, loud voices, louche crowds, and luxury prices— but a lovely view.
For views of the Capitol Building, the best is Charlie Palmer Steakhouse, but their roof is only open for private events. Across the river, the Sheraton Pentagon City has a rooftop pool with views of DC, the Pentagon, and Arlington National Cemetery, but no rooftop lounge or restaurant.