I’ve been collecting websites that help with quests and such.  Here are some of the ones I use most often.

Items List -Niwa has a great list of all collectable items in game and where you can find them.
http://blog.sanriotown.co.id/niwaexia:hellokitty.co.id/category/list-item/

Quests - Another of Niwa’s posts, has a quest list.  Unfortunately not in English, but it’s good for checking to see if you have any quests left in a city.
http://blog.sanriotown.co.id/niwaexia:hellokitty.co.id/category/quest-guide/

Maps - Shows NPCs
http://wiki.aeriagames.com/hko/index.php?title=Category:Maps

Quest Guide - Good for beginners, this site helped me a lot when I started to play.  Only goes halfway up through London.
http://blog.sanriotown.com/videogamer:hellokitty.com/2009/09/19/quest-guide-for-hello-kitty-online-by-rikki21/

And of course SanrioTown!

Maybe one day I’ll make my own help website… but these are good sources for now.  :)