In no particular order..
Scott Westerfeld's Midnighters Trilogy
-Books: The Secret Hour, Touching Darkness, Blue Noon
This trilogy is science fiction. Westerfeld is one of my favorite authors, and with good reason. The series is about a small group of teenagers who were born at the stroke of midnight. Because of this, they have the 'privilege' of experiencing a twenty fifth hour of the day. Of course, there's a catch.
T. A. Barron's The Great Tree of Avalon Trilogy
-Books: Child of the Dark Prophecy, Shadows on the Stars, The Eternal Flame
-It's been a while since I've read this. I remember the intimidation I felt when faced with the sizes of these books. However, I did brave it and read them. I am very glad that I did. This book is fantasy, and it's about this, obviously great, tree. The tree is the life's blood of the creatures that inhabit it, and the books take you through a few threats the tree is faced with and the adventures that result.
Scott Westerfeld's Uglies Trilogy (or Quartet)
-Books: Uglies, Pretties, Specials, Extras
-This book is set in futuristic times. The world created is incredible, and the book causes you to think about society's twisted view of the typical 'pretty' face. I don't want to say any more, or else I'd ruin it by butchering the summary.
Tamora Pierce's Song of the Lioness Quartet
-Books: Alanna: The First Adventure, In the Hand of the Goddess, The Woman Who Rides Like a Man, Lioness Rampant
-This series is amazing. In a fantasy world where all knights are male and women are weak and fragile, Alanna rises up. Disguised as a boy, she trades places with her twin brother in order to train to be a knight. The quartet tells of the obstacles she faced in training and as a Lady Knight.
Caroline Cooney's Time Travel Quartet
-Books: Both Sides of Time, Out of Time, Prisoner of Time, For All Time
-The ONLY non-fantasy/science fiction series on my list. It's romance, and heart wrenching. Annie, who is your everyday twenty first century-er, is thrown back into time, where she meets the love of her life. The whole quartet is about fate playing with her, taking her back in time to find someone she loves only to be shoved back into present time. Many problems arise from this time travel, such as Strat, the love of her life, being locked in a loony bin while Annie is in an entirely different time period.