gyutae, I'm not surprised at all that you or many others don't know about Dragonforce, Lordi, Nightwish or Children of Bodom. Here is a bit of info on each band.
Nightwish - A european band formed in Finland in 1997. They are a symphonic metal band. My favourite band at the moment, definately worth a try. They recently fired their lead singer due to the fact that she was only in it for fame and fortune.Amazing vocals,lyrics and sound. Sometimes they even incorperate symphonies for a song or two. Nice for the metal/mystical fan.
www.nightwish.com
Children of Bodom - Also a european band formed in Finland.If you like black metal then try them out. Their guitar is what usually captures the audience, played by Alexi Laiho.I do not recommend this band to anyone who dislikes semi-understandable screaming.The drummer, Jaska, is absolutely crazy on the kit, but not so crazy as to not understand. He uses a lot of neat fills and double bass kicks his way through songs and concerts.
www.cobhc.com
Lordi - yet again a european band who recently won the Eurovision Song Contest. They play hard-rock and dress in weird cornyish monster costumes, but honestly, you try playing with long claws and a latex mask. When they perform live, the vocals in my opinion aren't great but the recordings are. If you like KISS then you'll like Lordi.
www.lordi.org
Dragonforce - by far the fastest speed/power metal band I've ever heard, ever. Formed in London, England, this band strives for pure speed in all their songs. Lead guitarist Herman Li just flies on the fretboard which will take anyone by surprise. I love the vocals and the fast-paced drumming.
www.dragonforce.com
I'm really into bands that people aren't familiar with. Try searching for a weird genre, like opera metal and you might find something worth listening to.
I hope that helps answer your question. Have fun :P