Some of these have been mentioned already, but I'll second them because ds12 and dd10 read them so much:
Asterix
Tintin
Calvin & Hobbes
You might also want to look at:
Pogo, by Walt Kelly - I loved it as a kid, and my kids fell in love with it this past summer; lots of books, but many are out of print (available used via Amazon though)
Bloom County, by Berke Breathed
Dilbert
Jellaby
The Tick
Moomin, by Tove Jansson (the comic strips collected in book form, not the chapter books)
Bone, by Jeff Smith
PS 238, by Aaron Williams, about an elementary school for kids with superpowers
Will Supervillains Be on the Final? by Naomi Novik (author of the His Majesty's Dragon series) - I think it's also about kids with superpowers
Then there are, as previous posters said, comic-book or graphic-novel renderings of lots of kids' books. Dd liked the comic-book versions of Orson Scott Card's Ender's Game and Ender's Shadow and the graphic-novel versions of James Patterson's Maximum Ride series; ds has a graphic-novel version of one of Terry Pratchett's Discworld novels but seems to prefer the text version.
