As of Rule Change 14 (07/11/12), in addition to post game prizes there's now a chance to get TREASURE BALLS
Treasure Balls are here! At the end of a match you may get a normal Post-Game Prize, or, even better, you can get a Treasure Ball! Treasure Balls contain a single item that could be something awesome, something amazing, or something truly unique! We have added a number of items that can only be received via Treasure Balls. Gotta catch them all!
There are 2 types of Treasure Balls each with their own sets of items. Only cosmetic items are in Treasure Balls. No Endorsements or Products. They drop at a different rate than Post-Game Prizes. You will still get Post-Game Prizes at the same rate. Now you may get an additional prize of a Treasure Ball when you normally would not get a Post-Game Prize The drop rate of Treasure Balls is considerable slower than Post-Game Prizes. Everything inside the Treasure Ball is worth the price of the key or better. In a lot of cases, it's better. Keys and Treasure Balls either are or will be tradeable soon. I'll have to check with UberGaf on the current status of that. Only the items listed in the Rule Changes as "Treasure Ball Only" are only received through a Treasure Ball. Everything else is still winnable via Post-Game Prizes. These are the five All-Star Mustaches and the three "The Biscuit" heads.
I asked Uber on twitter if any Classics can show up in Treasure Balls and they said no, so I'm curious what the super rare or unique items are.
Treasure Keys to unlock the Treasure Balls are $2.50 in the store, which seems fair to me, but I've seen people saying the would want them down to $0.99. I think it's a neat way to give Uber a little extra income, or fill your inventory with things to trade away people might actually want, instead of tons of useless products and extra endorsements nobody will want anyways. Some people just seem angry they're even attempting to add this, and saying TF2 a lot through gnashed teeth.
Anyone opened one yet? I've got a Helios one but I'm not sure if I should open it or wait to see when they become tradeable and what kind of value they have.