The most obvious sock reference is in Chamber of Secrets, when Dobby is freed by being handed a sock. In this case, the sock definately represents freedom. I also believe that Dumbledore's sock reference when he's talking to Harry in P/SS (after Harry goes alone to the Mirror of Erised), Dumbledore, while not being entirely honest, uses the socks comment to represent freedom. In a way, Dumbledore seems sad and almost regretful that he's in the position he's in: having to watch Harry go through what he goes through, being responsible for so many people, etc. When Dumbledore says that he wants socks, but people insist on giving him another book, I think Dumbledore really has a longing to just be a normal wizard, not "the only wizard Voldemort ever feared", therefore obliging him to be Headmaster at Hogwarts and bear the weight of the responsibility so heavily on his shoulders.
Naturally, there are other sock references. When Dobby gives Harry his socks for Christmas, those seem to be used only as a means to let Crouch/Moody find out that Harry is friends with Dobby the house-elf, which we find out Crouch/Moody uses to his advantage when he lets Dobby overhear him talking to McGonagall, knowing that Dobby will go straight to Harry.
Also, when Harry puts the Sneakoscope in Uncle Vernon's old sock to keep from being bothered by it (because, as we find out later, Peter Pettigrew is in the room with Harry), that is actually a clue that Peter's (or somebody's) around and up to no good.
That's all I got for now, but there are more, I just can't remember them...
Also, in chapter 13 of just about every one of the books, a MAJOR hint or character major to the top storyline shows up. This is the chapter in which the trio learn about Nicholas Flamel in P/SS, when Harry is taken into the diary's memory in CoS, when Sirius Black broke into Gryffindor Tower using Neville's list of passwords in PoA, when we get our first good look at "Mad-Eye Moody" in GoF, and when Harry has the detention from Hades with Umbridge in OotP. Also, it's worth noting that the "double 13", or Chapter 26, is when there is another major turn in the story of OotP. Harry's interview with Rita Skeeter appears then, and the atmosphere of the school seems to go back to Harry's favor. It's also the chapter in which Harry "witnesses" the confrontation between Voldemort and Avery and finds out that Rookwood has told Voldemort where to get the weapon. This leads to the confrontation between Snape and Harry in which Snape realizes for sure that Harry is "seeing" through Voldemort.
