As of November 2015, to find out which congressional district you live in, use the free tool available on the House of Representatives website at House.gov. Search your ZIP code to find out the name of your district, the name of your representative and the party to which they belong.
On House.gov, enter your ZIP code in the search box titled Search Your Representative at the top right corner of the page. On the search result page, the name of your district is to the left of the map of the district that corresponds to your ZIP code. The official portrait of your representative is below, with contact information to the right, including a link to your representative's official website and an email link to contact them.
Use your mouse or the plus and minus zoom buttons on the map to explore your district. Double-clicking on another part of the map outside of your district, however, does not re-center the map or load information for that different district. To search for another district, you must use the search box again at the top of the page.
Alternatively, use the map found on GovTrack.us. Click the Members of Congress link on the home page to access a search box that enables you to search by address or just ZIP code. Unlike the map on House.gov, the GovTrack.us map displays a color-coded overlay that names each district in the United States that you can explore using your mouse.