The Divergent series by Veronica Roth is quite popular right now:
I highly recommend this young Sherlock Holmes series by Shane Peacock:
http://www.theboysherlockholmes.com/
Andrew Lane is also working on a young Sherlock Holmes series:
Anything by Rick Riordan (Percy Jackson series) is worth a look:
http://www.rickriordan.com/home.aspx
James Dashner (The Maze Runner) is a good choice:
Try Suzanne Collins (Hunger Games) if you like the movie and haven't read the books yet:
http://www.suzannecollinsbooks.com/
If you like sports, give Mike Lupica's books a try:
http://www.mikelupicabooks.com/index.html
If you liked Gary Paulsen's Hatchet, try some of his other books:
http://www.randomhouse.com/features/garypaulsen/
Try Ally Condie's dystopian novels, Matched and Crossed:
Orson Scott Card's Ender's Game is a popular Sci-Fi choice for YA readers:
http://www.hatrack.com/osc/about.shtml
Tolkien's Lord of the Rings will always be the fantasy classic:
http://www.planet-tolkien.com/
Cassandra Clare's Mortal Instruments series is a popular choice:
http://www.shadowhunters.com/mortalinstruments/
Brandon Sanderson has written some great books, including The Mistborn Trilogy, The Alcatraz Series, and Elantris:
http://www.brandonsanderson.com/
Brandon Mull's Fablehaven Series, Candy Shop War, and Beyonders are possibilities:
Harlen Coben's YA mystery series with Mickey Bolitar is awesome (some mature content):
Scott Westerfeld (Uglies, Pretties, Specials) has some good YA science fiction:
Stephanie Meyer's (Twilight Saga, The Host) Website:
http://www.stepheniemeyer.com/
Darren Shan's Cirque Du Freak is a popular YA horror series:
Check out John Flanagan's Ranger's Apprentice: