Party Hats are head items which are given during Anniversary Parties. They are always pointy at the top and covered in different colored stripes. Party Hats are usually found at the Coffee Shop, where all players can obtain them. All of them are available during the time line of the party - which usually lasts for a day or a few days - and are never available again. The only exceptions are the 5 Year Celebration Hat and French 5th Year Party Hat, which are currently unlockable.
The 6th, 7th, and 8th anniversary hats have had some relevance to the respective years they represent in Club Penguin. The 6th Anniversary Hat was orange, green, black, and purple because it was the first Anniversary Party to take place at the same time as the Halloween Party. The 7th Anniversary Hat had the colors of Club Penguin's new logo that was revealed in the same month. The 8th Anniversary Hat has the colors of the Rainbow Puffle, because it released earlier that year.
Party Hats are a good way to determine a penguin's rarity.