Bad Machinery is currently my favourite webcomic. It follows the adventures of groups of schoolchildren in the fictional, supernatural-infested Yorkshire town of Tackleford, as they go about attempting to unravel mysteries.
John Allison, the comic's creator, has said he thinks of himself as a writer who's learned how to draw, but his illustrations are just as witty and clever as the dialogue or plotting, all of which regularly make me laugh out loud. He has been making webcomics for 14 years now - longer than almost anyone else - so he has had plenty of time to master the craft. His first webcomic, Bobbins, started in 1998 as single strips with art that looked like it had been made in MS Paint, but not in a bad way. In 2002 he started again with Scary Go Round, using many of the same characters, and that ran for seven years - during which time the art went through two or three major shifts of style before settling on the same hand-drawn line-art with flat colours that we see in Bad Machinery. The writing also improved fairly steadily over that time.
Bad Machinery started in September 2009, just a week after Scary Go Round finished. In comic-time, three years passed in between - giving Ryan from Scary Go Round time to get married and become a secondary school teacher, and for several minor characters from the earlier series to grow old enough to be in his class.
The idea of child sleuths is one with a long history, of course, but I've never known it to be done with so much humour, or with such a sharp eye for the habits and language of the youth of today. The characters are strong and interesting, and we get a real sense of the kids growing up as time goes on. The plots, though silly, are substantial enough that the mystery aspect of the comic doesn't get lost under all the humour.
Bad Machinery is currently being updated four times a week, Monday through Thursday. I recommend you start here, read until you're up to date, and then go back and read all of Scary Go Round and maybe Bobbins as well.