I'm on the 6th book, The Half Blood Prince now.
Why hasn't anyone tried to kill Harry Potter until now? (Except Voldemort..)
UPDATE: The question was meant to address someone trying to kill Harry Potter for being Harry Potter (The Boy Who Lived/Chosen One from the Prophecy), and not merely for being in the wrong place at the wrong time.
Well, in book one Professor Quirrell tried to kill Harry. In book two, the basilisk tried to kill Harry. In book four Barty Crouch Jr. disguised as Mad-Eye Moody tried to kill Harry.
And book seven spoilers (DON'T HOVER OR HIGHLIGHT IF YOU DON'T WANT TO KNOW):
In book seven Vincent Crabbe attempts to kill Harry with Avada Kedavra at the end of the book in the Room of Requirement chapter.
These examples are attempts on Harry's life on behalf of Voldemort, but other individuals are carrying out the deeds.
ETA: One might argue that Dobby also attempted to kill Harry throughout book two ;)
ETA 2: Aragog's sons and daughters tried to kill Harry and Ron in book two. The Whomping Willow might have managed it, but we don't know its intentions, again in book two. I believe in book five, Harry, Ron, and Hermione almost were shot with arrows by the centaur herd (going off the top of my head on that one). Professor Lupin, as a werewolf, came after Hermione and Harry in book three. A Dementor almost killed Harry in book three by causing him to pass out in the sky and fall over 100 feet down to the Quidditch pitch; however, Dumbledore saved Harry with Arresto Momentum. If I think of any more, I'll edit them in. :)
Aside from the people who have tried to kill Harry Potter (See Slytherincess's answer), Pre-book 4, there was no real need to kill Harry, as Voldemort was dead. After that book, Voldemort made it quite clear that he was the one to kill Potter. Thus, no one would kill him besides Voldemort, although there were many people who attempted to capture him.
Since he has survived the most dangerous spell from the most deadly wizard, I don't think anyone else would dare trying.
Also, as mentioned, Voldemort forbids anyone to try, claiming the right to kill HP himself.
Besides, he is the center of attention from the very beginning, often surrounded by numerous witnesses and powerful wizards.
Even if I were a evil wizard, I would keep my distance to that kid, even less try to kill him. ;]
Well, I guess many assumed that since he survived the killing curse he must've been a exceptionally gifted wizard, or even a potentially Dark Wizard in the making. You don't want to mess with Harry Potter! Moreover, most people must think Dumbledore placed unimaginable magical protections around him when he's not at Hogwarts. It's even Possible there were counfounding charms, repelling charms and suchs, thought we know there were wizards around such as Dedalus Diggle, and Mrs Figg kept an eye on Harry.
Again the problem is most people would only think about using magic on him, but none would expect a muggle attack. I'm not even takling about a sniper rifle killing him, but a muggle under the imperius curse ordered to kill him by any means, a teacher at school, some of the kids in Privet Drive, a "rabbid dog". A poisonous handshake, or a poisonous dart would be just as effective.
