If you like The Diary of a Wimpy Kid, try these:

Amelia's Notebook series, Marissa Moss (Betweener Fiction)
Attack of the Fluffy Bunnies, Andrea Beaty Jr. Fiction)
Attack of the Mutant Underwear, Tom Birdseye (Jr. Fiction)
Babymouse series, Jennifer Holm (Jr. Illustrated Novels)
Clarice Bean series, Lauren Child (Betweener Fiction)
Clementine, Sara Pennypacker (Betweener Fiction)
The Day My Butt Went Psycho, Andy Griffiths (Jr. Fiction)
Dear Dumb Diary series, Jim Benton (Jr. Fiction)
Diary of a Stinky Dead Kid, Stefan Petrucha, (Jr. Illustrated Novels)
Dork Diaries, Rachel Renee Russell (Young Adult Fiction)
Ellie McDoodle; New Kid in School, Ruth McNally Barshaw (Jr. Fiction)
Franny K. Stein series, Jim Benton (Jr. Fiction)
Frindle, Andrew Clements (Jr. Fiction)
The Homework Machine, Dan Gutman (Jr. Fiction)
Hank Zipzer series, Henry Winkler and Lin Oliver (Jr. Fiction)
The Invention of Hugo Cabret, Brian Selznick (Jr. Fiction)
Justin Case; School, Drool, and Other Daily Disasters, Rachel Vail, (Jr. Fiction)
Little Wolf series, Ian Whybrow (Jr. Fiction)
My Weird School series, Dan Gutman (Jr. Fiction)
No Talking, Andrew Clements (Jr. Fiction)
Stink series, Megan McDonald (Betweener Fiction)
Stuart’s Cape, Sara Pennypacker (Betweener Fiction)
Wayside School series, Louis Sachar (Jr. Fiction)