I will be gradually adding more links as I go along.

Australian Children's and YA writers:

Paul Collins and the Quentaris books (my title 'Pirates of Quentaris' was published in May 2006)
Meredith Costain
John Marsden
Sheryl Gwyther
Dale Harcombe
Robyn Opie
Jackie Hosking - Jackie also publishes the PIO newsletter for children's/YA writers
Sue Whiting
Sally Odgers - Sally also does manuscript assessments for children's and YA manuscripts, as does Di Bates at Enterprising Words
Sally Murphy - also has a website that reviews Australian kid's books

Overseas writers:

Kristi Holl - I first met Kristi at Chatauqua Writers' Workshop and she ran some great sessions on tension techniques and mysteries for middle graders. Her website has some excellent resources.
Hope Marston - I met Hope at Chatauqua Writers' Workshop too - check out her non-fiction. The Little Books have beautiful illustrations.

Sites about writing for children:

SCBWI - Society of Children's Book Writers and Illustrators - an international organisation for children's writers and illustrators that provides a wealth of resources. Members can download info on publishers and magazines, as well as lots of other handy stuff.