Mm, Sepultura is the obvious example, but there's loads of great Brazilian bands that just don't get any exposure outside of Brazil. One favourite is a band called Legiao Urbana ("Urban Legion"), though their quality is pretty dependent on the quality of the lyrics so unless you speak Portuguese I don't know how much you'll get out of it.

That said, go on Kazaa and download some of their acoustic songs - they did an absolutely brilliant MTV Unplugged session with most of the acoustic versions being better than the originals by a fair margin. I would recommend doing a search for "legiao acustico" and downloading anything that comes up.

Specific recommendations include "Indios", "Pais e Filhos", "Teatro Dos Vampiros", "Hoje A Noite Nao Tem Luar", "Eu Sei"
One of my favourite Brazilian bands is one called Skank (seriously

). Their music is a very cool mixture of rock, ska, reggae, and Brazilian beats - it's just so addictive and energetic. Download some of the following: "Partida de Futebol", "Garota Nacional", "Jackie Tequila", "Te Ver", "Tao Seu", "Resposta", "E Proibida Fumar", "Estivador", "Indignacao", "Esmola", "Eu Disse A Ela".
Another good band is the Paralamas do Sucesso ("Mudflaps of Success"

), with favourite songs including "Uma Brasileira", "Tendo A Lua", "O Amor Nao Sabe Esperar", "Trac Trac", "Lanterna dos Afogados", and "Lourinha Bombril", as well as the amazing live rendition of "Meu Erro" with Zizi Possi.
If you want something a bit more tongue-in-cheek (though I don't know how well the humour will come across to a non-Portuguese speaker...), try the Mamonas Assassinas (which basically translates to "Big Killer Boobs"). Especially "Vira Vira", "Bois Don't Cry", and "Pelados Em Santos". Great stuff.
Anyway, that should get you started.

Unfortunately I don't have nearly as much experience with Brazilian music as I'd like. Trust me when I say this is just the tip of the iceberg and I'm still trying to discover more.
Cheers,
Dunc
P.S. - If you want recommendations for some more laid-back, or at least non-rock Brazilian music, let me know.