What unusual effects can taunts come in

As nimcap answered, unusual hats are just hats with special particle effects. They have no stat benefits, only visual effects.

The only way to obtain them is to trade for them or to find one in a Mann Co Supply Crate. Crates have a low chance to contain an unusual hat.

According to data gathered by users on the forums, it seems that series 1 crates have about 2.45% chance of containing an unusual item, and series 2 crates have about a 1.5% chance. You can tell which series of crate you have by reading the tooltip.

To open the crate you need to buy a Mann Co. Supply Crate Key, which costs $2.49 in the store. If you'd rather not spend money you can trade the crate to someone. Items that come out of crates are craftable and tradeable.

You can find more details about the drop rate on the TF2 wiki page about crates. Keep in mind that this data is gathered by players and might not be totally accurate, but it's probably in the ballpark.

From two helpful people on the Steam forum, here is a list of particle removals and optimisations.

You probably want 'Direct X 9 crit boost file by itself', unless you know you're running DirectX 8 (usually in conjunction with a config). I think that it still removes the halo effect though.