Rolling Stones, either "Brown Sugar" or "Start Me Up" (since they're roughly inverses of one another). Guess I give the nod to BS, because I heard it first and SMU loses points for being used in a Microsoft ad.
Second the nomination of Yes "Roundabout." Also, "Siberian Khatru" from "Close to the Edge"
Others that come to mind:
Santana, "Europa (Earth's Cry, Heaven's Smile)"
Hendrix, "Little Wing" and "Purple Haze"
Buffalo Springfield, "For What It's Worth"
Jefferson Airplane, "Volunteers"
Neil Young, "Rockin' in the Free World"
Mountain, "Mississippi Queen" (gotta love The Great Fatsby)
Ten Years After, "I'd Love to Change the World"
Beatles, "Paperback Writer"
Paul Simon, "Late in the Evening"
Eric Clapton, "Layla" (or about half of the other songs on that album...)
And on and on ... isn't music great? Happy listening!