I'm actually copying and pasting part of my answer to yesterday's Vintage items question here...
Unusual hats were added to the game during the Mann-conomy update on September 30th.
These are hats have particle effects attached to them, such as ghosts or a sunbeam. The names for these items are shown in purple. They are not available through the random drop system, but instead as a rare drop from a Mann Co. Supply Crate, which require you to buy keys from the Mann Co. Store (for $2.49 each) to open.
Of course, like all hats, they can be traded. Expect to trade a lot of hats for one, though, as they are exceedingly rare.
If you are trading to try and get Unusual hats, note that items can now be renamed... check the name color (should be purple for Unusual items) to make sure it really is an Unusual hat.
Also, there have been scammers out there lately who will put in an item and then change it while you are putting your items into the trade window. Always double-check what items are in the other person's side of the trade window before accepting.
The list of available Unusual hat effects are:
- Haunted Ghosts
- Circling TF Logo
- Circling Heart
- Circling Peace Sign
- Massed Flies
- Green Confetti
- Purple Confetti
- Green Energy
- Purple Energy
- Burning Flames
- Scorching Flames
- Searing Plasma
- Vivid Plasma