Demon Hunter
This band plays rock and heavy metal and in my experience people who hear them without knowing it would never guess they were Christian. Have a listen to some of their music:
Thousand Foot Krutch
Hailing from Canada, some of you may or may not know this band, as they have a larger following in the USA. To some's surprise, Trevor McNevan, founding member, guitarist and lead vocals, has close ties to Three Days Grace, Finger Eleven, and Breaking Benjamin. He even had a hand in Three Days Grace's demo. You can pick up anyone of their albums and every song is crazy awesome, so it is hard to pick some favourites.
P.O.D.
Here is a great band that has been around for a long while and have great mainstream success. You probably know them, so I just let you get to the music. If you don't recognize either of these two songs, look up "Boom".
Pillar
This is one of the bands I've found on people iTunes and they had no idea they were a Christian band. I couldn't find a good quality video that had embedding enabled or wasn't blocked in Canada (don't know how record companies expect people to spread music around), so just click "Frontline".
Here is a quick run down of some more great Chrisitan bands you may already be listening to.
Skillet
Relient K
Project 86
Decyfer Down
I could go on with many more great bands, some unknown and others mainstream artists, but why not just discover them for yourself. Here are two free sites streaming Christian rock.
Or you can sign up for New Release Tuesday or Hear It First. Both of these sites give you several free downloads each week, offer full album previews, contests and NRT even lets you earn points for free swag.