No one has posted here for a while, and I have some wonderful series I like to rave about.
Meghan Whaller-Turner's "The Thief", "The Queen of Attolia" and "The King of Attolia"
Scott Westerfeld's "Uglies", "Pretties" and "Specials"
Eoin Colfer's "Artemis Fowl"
Jonathan Stroud's "Bartimaeus Trilogy"
Louise Rennison - the Georgia Nicholson series. The first is called "Angus, Thongs and Full Frontal Snogging", and the other titles are just as outrageous. Hilarious reading if you like Brit humor (the MC is 14 so not for younger kids)
I enjoyed Angie Sage's "Magyk" and "Flyte"
Loved Richard Peck's "A Long Way from Chicago" and "A Year Down Yonder", but I guess 2 books don't count as a series ...
Margaret Peterson-Haddix has written a lot of "Among the Hidden" sequels (6, 7 , 8?) The first is good, but they go down-hill from there. Kids seem to love them despite the writing.
Jeane duPraue (I think that's how you spell her name) wrote "City of Ember", then "People of Sparks", and recently "Prophet of Yonwood". Again, the first book is great but couldn't get into the others.
(Wow, Sam - all the Disk World books. That takes some staminer, tho I love Terry Pratchett)
Will add as I think of more ...