Ohh, definitely want to check out Common Market, Blue Scholars are dope. I'll post a few that haven't been brought up yet, pretty good suggestions so far though.
Eyedea (Kinda like Atmosphere, but it's new and different)
Qwel (Probably the best lyricist I've ever heard, and his message is so on-point)
Aesop Rock (Badass flow and voice, I'm a fan)
Jurassic 5 (You've probably heard of them, but get more... they're SO fucking talented...)
MF Doom (Really unique)
Soul Position (Cool group with songs about shit that matters)
Typical Cats (Frontman is that guy Qwel, absolutely incredible)
Del/Hieroglyphics (Del is just amazing. My favorite rapper. Get his shit)