Traditional: Bill Monroe (created it), Flatt and Scruggs, Stanley Bros., Del McCoury Band

New-Grass: New Grass Revival, Yonder Mountain String Band, Railroad Earth, Sam Bush Band, Tony Trishka, The Seldom Scene

Poppy-ish: Nickel Creek
If you are just getting into it I would hit the easy going stuff first. Gillian Welch is a good start. She is more folky but her guitarist is fricking beast. Seldom Scene is a litle more mainstream. After that you can move up to Ricky Skaggs. Becareful you don't get Skaggs' pop country garbage! He went jiggy for a while but then came on back home to playing grass. Then I would go for the hard stuff. Country Gentlemen, Ralph Stanley & Bill Monroe. That should get you going.
Quote by Fallon, Jones & Sage Francis

"By the way my name is Marcus. But if you like you can call me Sensei."

American Idol - or as I call it, the Gong Show Rip-Off

According to #Yeezuz, Kanye West is a god. As if I needed more reasons to be an atheist.
Dude, definetely get Ralph Stanley's song: Rank Stranger. It is fairly easy and it is an AWESOME SONG! YEEEEEAHHHHHHHHH!!!!!!!!!!!