The “official” Gershwin website. It includes a jukebox, biographies of Ira and George, anthologies of their film and and show music, lots of good clips of music in the jukebox, with information about performers and the song.
http://www.gershwin.com/

The official website of George and Ira Gershwin. This site includes an extensive bibliography of the writings by and about the Gershwins. The biographies are brief, but there is an excellent photo gallery and discography.
http://www.cmgww.com/music/gershwin/