1. Hunger Games Trilogy by Suzanne Collins
Although these books are "popular" right now, the quality of writing is actually good, and the story line is positively gripping. I'm pretty sure even if you hate the book, you'll have to keep on reading because of the suspense.
If you don't know what these books are about, they are a dystopian series following Katniss Everdeen. In this series, some sort of great war has shattered the U.S. as we know it, and broken it into thirteen districts. Katniss lives in district 12, the poorest district of them all. These districts are harshly ruled by the government, and the people are kept under the control of its iron fist.
A manifestation of this iron fist is the annual "Hunger Games"--which is reminiscent of the movie Gladiator, except for children. In these games "tributes," a girl and a boy from each district, are chosen to go to the games and fight to the death with the other tributes in a large nationally televised event. Traditionally, only one person wins, and the rest are killed in the process. When Katniss' young sister, Prim, is chosen as tribute, Katniss volunteers herself instead in an act of sacrificial love which becomes a theme in the books.
I would recommend these for older kids. By older, I mean kids who can handle sometimes graphic violence, and also make sense of the allegorical plot (of course, if you are older than an "older kid," knock yourself out--these books are fantastic). Although there are points with violence, the author, Suzanne Collins, writes with a definite sense of purpose and the themes of these books can be mulled over for days because of it. And, the movie is coming out! 12 more days!
2. Alex Rider by Anthony Horowitz
The Alex Rider series is about an orphaned teenage boy whose uncle was recently killed on a mission for the MI6 (British intelligence). The MI6 eventually contacts Alex and tells him his uncle had been raising him to be a future spy (martial arts, scuba diving, multiple language lessons, etc.)--and that they need him to work as a field agent.
And so begins another addicting, action packed series. I read these in the seventh and eighth grade, but just recently finished reading the final book, Crocodile Tears. The books are fun and engaging--kind of like Mission Impossible or James Bond, only in book form. There is not really any foul material in these books, other than some perilous situations and occasional violence, so I would recommend these to all interested parties--well, within reason, of course.
3. Maximum Ride by James Patterson
Having said all that (it's a mouth full... sorry), these books follow the adventures of fugitive mutant children (and by fugitive, I mean children with 98% human DNA and 2% avian). This band of children, having broken out of the sinister labs that created them, start out living in a secluded home in the mountains. Their tranquil lives are interrupted (shattered, really) when 'erasers', mutant creatures made by the same scientists, arrive and are on the hunt for the flying kids. The band of bird children flee, and from then on are constant fugitives, running from those who created them.
Combine all that with some "save the world" action, and you've got an interesting series suitable for sixth grade and up. Of course, the target audience (I think) is made up of preteens and teens, so maybe more like middle school and up? I don't know. If it helps, James Patterson writes that he wrote these books free of foul language, and other "bad" things in order to keep these books PG rather than PG-13...