Tom Petty is a classic. One of my favorites to listen to. I can be in any mood and it sounds great.

UB40 is a great reggae band (Red Red Wine is probably their most famous song)

Toby Keith, Brad Paisley, Tim McGraw are all great country artists.

The Roots have some good songs as other mentioned, and Zion I as another member mentioned is a great rap band as well.

I'd also suggest some Dave Matthews band, Citizen Cope, Flipsyde, Atmosphere, Jedi Mind tricks, Jurassic 5, Living Legends, Red Hot Chili Peppers....