His most famous (and commercialized, and copied, and parodied) piece is:
The Revolution Will Not Be Televised
But some less well known pieces are:
Ghetto Code (Dot Dot Dit Dit Dot Dot Dash), an inspired meditation on the United States government's role in the world.
H20-Gate Blues, on the moral aftermath of the Watergate scandal. (I'm not certain if an honest-to-goodness sample was used, but the music and some lines from this piece also inspired KMD on the piece Bananapeel Blues which has nothing to do with Nixon, but is instead a breakdown of Five Percenter theology).
Wikipedia: Gil Scott-Heron
NPR Music: Gil Scott-Heron, Poet And Musician, Has Died by Daoud Tyler-Ameen