We learn that, in the past, the Dursleys have given Harry an old pair of Uncle Vernon's socks for his birthday.
The very first time we meet Harry he is in his cupboard looking for socks.
Dumbledore tells Harry that in the Mirror of Erised he sees himself holding a pair of thick, woolen socks.
Chamber of Secrets
At The Burrow, Mrs. Weasley is "fussing over the state of his socks."
And also at The Burrow: "Mrs. Weasley dashed about in a bad mood looking for spare socks."
When Dobby explains to Harry that, for a house-elf, clothes means freedom, he says: "The family is careful not to pass Dobby even a sock, sir."
Harry tricks Lucius into giving Dobby Harry's sock.
Prisoner of Azkaban
Harry uses Uncle Vernon's old socks to cover his Sneakoscope: "I never wear those socks if I can help it."
One of the worst birthday presents Harry ever got was that pair of Uncle Vernon's old socks.
Ron unwraps a pair of maroon socks for Christmas, but drops them to check out Harry's new Firebolt.
The Fat Lady is found hiding in a portrait of Argyllshire, a place famous for socks.
Goblet of Fire
At The Burrow, Mrs. Weasley washes Harry's socks.
On Christmas morning, Harry gives Uncle Vernon's "foul," "mustard-yellow," and "extra knobbly" socks to Dobby.
Ron gives Dobby an unwanted violet pair he got from his mum. Dobby is ecstatic. Ron (looking embarassed) says: "They're only socks..."
Dobby gives Harry a pair of socks; one red, with broomsticks, the other green with Snitches, that he knitted himself.
At the Yule Ball, Moody tells Harry ... "Nice socks, Potter."
Harry resolves to buy Dobby "a pair of socks for every day of the year."
On the morning of the first task, Harry dressed so inattentively that it was a while before he realized that rather than trying to pull on a sock, he was trying to pull a hat onto his foot.
While Hermione is arguing with Hagrid about S.P.E.W., Hagrid is putting on socks.
Harry and Ron have fun in Hogsmeade picking out strange-looking socks for Dobby, including "a pair patterned with flashing gold and silver stars and another that screamed loudly when they became too smelly."
Order of the Phoenix
Tonks talks about her mums knack of getting socks to fold themselves ... "Its a kind of flick-" She flicked her wand hopefully. One of Harrys socks gave a feeble sort of wiggle and flopped back on top of the mess in the trunk.
Mundungus's pipe had a lingering acrid smell of burning socks.
"Match his what?" said Mrs Weasley absently, rolling up a pair of maroon socks and placing them on Rons pile.
When Fred and George offer to attach Ron's prefect badge to his forehead with a Permanent Sticking Charm, Ron wraps it tenderly in his maroon socks and locks it in his trunk.
Seamus dressed at top speed next morning and left the dormitory before Harry had even put on his socks.
When Harry looks in the Pensieve at Snape's memories, he sees a group of laughing girls (including Lily Evans) sitting with their shoes and socks off, cooling their feet in the water.
Harry kept his Felix Felicis potion in a pair of rolled-up socks in his trunk (under which he also found his Marauder's Map).
The Weasley twins get Mrs. Weasley flowers, saying they appreciate her more now that they have to "wash their own socks."