Portishead is a really good band. The sound is really mellow but intense. I really like the singer's voice and sometimes even have an orchestra playing with them.
They have a good concert on dvd. It's "Live at the Rose Garden in N.Y." or something like that. It's a really good concert to check out.
Massive Attack is a cool and similar sounding band. I don't mean that they sound alike or anything, but they are in the same vein of genre so to speak. They also have good melodies and grooves.
Portishead is a KICK ASS BAND! Beats junk like Lacuna Coil and Evanessance any day. It's great for female vocalists to have less than typical role models to look up to. I also like Arch Enemy and Otep although both are more hardcore metal and less melodic.