jjimm, Portishead is not a duo. They are sweet, though.
Here’s some musical duos* I like:
Sleigh Bells - Loud, overblown modern pop with a guitar edge. The guy is the guitar player/beatmaker, the girl sings. The guy used to be in the band Poison the Well which is kind of a wild idea.
Beach House Baltimore represent! Dreampop duo, girl singer with a husky, smoky voice. Woozy guitar lines.
Death From Above 1979 A bass (sometimes keyboard, he went on to form MSTRKFT) and a drummer. These guys rock. Or I should say, rocked.
Crystal Castles - Once again, the girl is the singer + image maker, the guy creates the music. Indie rock that takes major cues from techno and the sound of video game music. Lots of manipulation of the vocals, generally a good driving beat.
Broken Bells - The singer/guitarist is from The Shins, and the one making the rest of the music is Danger Mouse, and they generally sound like a mix of the two.
Ratatat An instrumental tech rock group featuring a guitarist and a computer guy.
The Black Keys You’ve heard of these guys already, but they’re known for a reason.
The Raveonettes Danish duo that I remember listening to a lot when this song came out. Not so much since then, take that how you will.
The Knife This a Swedish artsy duo who wrote one of my favorite songs ever, linked here, with “Heartbeats”. Jose Gonzalez does a really splendid acoustic cover of this song. I don’t know how he figured it out.
The Blow - Once again it’s a quirky girl singing and a guy making music for her. Girls who like this band are generally more fun, in my opinion.
Two Gallants - Two guys, a guitar and drum. The drummer has a sort of melodic style that really stuck out in this song when I first heard it. They’d be great to see live.
The Chemical Brothers - Weird to think of a totally techno group as a duo (like it matters?) but it happens quite often (so it seems like it must matter). It probably has something to do with balance and perspectives, but these guys have been making great music for decades in a genre where the life cycles are often shorter than that of a fruit fly.
*All of these groups do refer to themselves as duos, but obviously sometimes need to employ outside help. The Raveneottes are a prime example - I think they even have the back-up musicians playing with them in their video.