A-Z Mysteries, and I strongly second the Henry and Mudge, Magic School Bus and Geronimo Stilton

Great suggestions here, and I also want to to the list: Fluffy the Guinea Pig. These are really silly early chapter books about a class pet who has a wild imagination. The author is Kate MacMillan. Good for girls and boys both.

And did anyone mention Cam Jansen?

I second Fluffy and Cam Jansen! Great suggestions Lorel!!! Fluffy is too cute - we own several of them - and we own a ton of Cam Jansen books too. I have 2 boys and they love both of these series.

Try hard to stay away from Junie B. Jones. They're awful but kids love them. Mine hadn't read them until he went to public school and the teacher read them to the class (1st grade). She thought they were "educational" because one was about Christopher Columbus. I try to steer mine away from them but sometimes he digs in his heels. It's not worth a battle, but I'd prefer he read something else.

Captain Underpants - yes there is some misspelling in the comic portions, however they are really strong on imagination and problem solving!

Frannie K. Stein, Little Girl Mad Scientist - My boy loves these, she is not too girly, it's mostly icky science fun.

Calvin and Hobbes collections- Fantastic level of vocab., for kids who love to figure out big words. Can be put down and picked up anytime. My son practically wears his Calvin and Hobbes books. And they are of course hilarious even for adults.

For more advanced reading level, but still a large serving of silliness: the Wimpy Kid books. There are only 2 in the series so far, but more are planned.

Couple more:
It's raining pigs and noodles - poetry collection by Jack Prelutsky
Anything Roald Dahl - for the more advanced reader...

I also second (third?) A-Z Mysteries and Magic Schoolbus (the books, not the videos, which I would not recommend). Our son has also liked the E. B. White Classics - Stuart Little, Charlotte's Web, and Trumpet of the Swan, though we help him with them a lot. They make great listening for long rides in the car. Also, as a listener, Harriet the Spy.