D-12, short for the Dirty Dozen, are a group of 6 rappers with two names apiece from Detroit. Their popularity has been given a boost by the incredible success of their executive producer and longtime member Eminem.
I like D-12 because they move beyond the standard bullshit that is general rap music. Their songs don't just sound good, they present interesting viewpoints on volatile issues, Furthermore, if one can look beyond the violent and perverse "characters" the group's members play on the album, they make really good points about life in general: always give respect where it is due, stay true to your friends, do not lie.
For an example of the kind of legitimate yet controversial argument they make, take a listen to "Revelation," a track in which they defend the anger and frustration that was felt by the Columbine murderers. This is a good point; although the Kliebold and Harris were definitely wrong to kill their classmates, we need to understand what they were feeling or it will happen again.