I second Relient K, they are great.
Here are some bands that are extremely good:
Burden of A Day: Hardcore with screamo but they are extremely guitar filled.
Thrice: Amazing stuff but I recommend you check out their album Vhiessu first.
Relient k: Punk mainly but their early stuff is awesome.
Angels and Airwaves: Very space rock sound but their stuff is awesome.
Anberlin: Pretty good, not my favorite but their stuff is decent.
Blindside: Hardcore band like Burden of a Day. If you really like screamo, check out their album A Thought Crushed My Mind.
Coldplay: You probably have a vendetta against these guys since it's so obvious how many people listen to them.
Emery: A hardcore band like Burden of A Day
Ivoryline:An rocker band. They are good and there new album was great.
Lights: If you like techno, she's good. Canadian too.
mewithoutYou: More than likely you will hate this band from the moment you listen ot them as they speak their lyrics instead of singing them. Their lyrics and music however are astonishing and they are one of my favorite bands.
Paramore: If you havn't already listened to them or heard of them, then these guys are pretty good.
Secret and Whisper: An amazing band that it is really hardcore without screaming.
Sent by Ravens: Relatively new band coming out that has an album coming out next year. Get their song Trailer Vs. Tornado
Skillet: Hardcore band, their stuff is really good but I would recommend getting their album Savior first.
Switchfoot: Pretty good band, Oh Gravity was their latest album and it was good.
Underoath: Hard, hardcore band with a lot of screamo. If you don't like that, don't get into these guys. Their good though.
The Wedding: Decent band, not great but still pretty good. Get their song Return from the album The Sound The Steel.
Wavorly: A pretty good band that incorporates a lot of strings in their music. They rock though on the one song Time I Understood.
Sum 41: Once again if you haven't heard of these guys you probably should listen to their early stuff. Their latest album Underclass Hero was sensational though.
Those are some bands that I know of and I listen to pretty much all of them.
If I had a chance for another try, I wouldn't change a thing.
Cause it's made me all of who I am inside.
And if I could thank God, that I am here, and that I am alive.
And everyday I wake, I tell myself a harmless little lie....
the whole wide world is mine