Too many older artists to name. However, I'm shocked that no one mentioned Bob Dylan.
I'm going to list some modern artists who are pretty much legends or will be someday:
Jay Z: He cemented his legend status a long, long time ago. Is one of the most respected figures in one of, if not the most dominant genre of music today. "99 Problems" and "Empire State of Mind" are the kind of songs even soccer moms know.
Eminem: He's the hip hop Elvis, so he was bound to be considered a legend based on that alone. One of the most loved and respected rappers in the hip hop genre, and obviously has huge crossover appeal.
Beyonce: She was already an icon before, but her last two albums, IMO, raise her prestige even more. Before, you knew her by her singles. Now, it's clear that she can make more artistic statements with her albums as well.
Justin Timberlake: Pretty much cemented himself as being multi-talented. His sporadic music appearances are still big hits as well.
Kanye West: He has the most acclaimed album of the decade so far. Pretty much loved by most music critics at this point. The controversy only adds to his myth. Its clear that he's the big influence on the current generation of rappers.