Unusuals come from only one place: Unboxing. You have a fairly small chance every time you open a crate or case to get an unusual. What unusual you get is determined by what crate you've opened and if it's halloween or not.

Special short term hats like the Thirst Blood only get to pick from basic generation effects, but get to pick between all of them.

Gen 1:Crate Series #1-#25
Gen 2:Crate Series #26-#57
Gen 3:Crate Series #59+ hat crates, excluding #87 which is a restricted case.
Ween Gens only during the annual Halloween event.

Halloween effects from basic crates only during the halloween event.

These can only be unboxed from their associated crate.
Gen 1 from Robo Community Crate Series #58
Gen 2 from End of the Line Community Crate Series #87
Gen 3 from Quarantined Collection Case or Confidential Collection Case
Gen 4 from Gargoyle Case

These can only be unboxed from crates with taunts as loot. Ween Gens only during the annual Halloween event.

This was the first set of unusuals, released with the Mann-Conomy Update in September 2010. It came with 14 unusuals, and every crate series from #1 to #25 only recieve hats with these effects, unless it's halloween.

• Burning Flames
  
• Scorching Flames
  
• Sunbeams
  
• Vivid Plasma
  
• Searing Plasma
  
• Purple Energy
  
• Green Energy
  
• Haunted Ghosts
  
• Purple Confetti
  
• Green Confetti
  
• Circling Heart
  
• Circling Peace
  
• Circling TF Logo
  
• Massed Flies

Burning, Scorching, and Sunbeams are the most sought-after of this group.

On August 18th in 2011 the second set of unusual effects were released. 8 unusal effects that can be unboxed in crate series #26 through #57, except during halloween.

• Stormy Storm
  
• Blizzardy Storm
  
• Smoking
  
• Steaming
  
• Bubbling
  
• Orbiting Fire
  
• Orbiting Planets
  
• Nuts n' Bolts

On June 19th in 2013 the so far final basic generation of unusual effects came out. 7 effects that could be unboxed from any hat-crate from #59 onwards as long as it wasn't halloween.

• Cloud 9
  
• Disco Beat Down
  
• Miami Nights
  
• Kill-a-Watt
  
• Terror-Watt
  
• Dead Presidents
  
• Aces High

Starting with the third Scream Fortress halloween update the halloween effects were unboxable only during the halloween event, but no basic generation unusual effects could be unboxed.

• Cloudy Moon
  
• Cauldron Bubbles
  
• Eerie Orbiting Fire
  
• Flaming Lantern

On October 26th in 2012 the second series of Halloween-only unboxable effects came out. Like the previous series they were unboxable only during halloween's event and replaced the normally possible unusual unboxings.

• Harvest Moon
  
• Misty Skull
  
• It's a Secret To Everybody
  
• Stormy 13th Hour
  
• Knifestorm

The next set of halloween unusuals was released on October 29th 2013. Like the two previous series they were unboxable only during halloween's event and replaced the normally possible unusual unboxings.

• Arcana
  
• Spellbound
  
• Hellfire
  
• Demonflame
  
• Darkblaze
  
• Something Burning This Way Comes (Burning Bat)
  
• Poisoned Shadows (Eerie Bat)
  
• Chiroptera Venenata (Scorching Bat)

This set of unusual effects were released on October 29th 2014. Like the previous series they were unboxable only during halloween's event and replaced the normally possible unusual unboxings. Since at this time Taunts had been given unusual effects, 2 taunt effects were also included.

• Bonzo The All-Gnawing
  
• Amarinthine
  
• Stare From Beyond
  
• The Ooze
  
• Ghastly Ghosts Jr.
  
• Haunted Phantasm Jr.

• Ghastly Ghosts
  
• Haunted Phantasm

On October 21, 2016 the Halloween event began and the effects from that year started dropping when you uncrated. Unlike previous Halloweens, every unboxed unusual during the event was guaranteed to have an effect from this year's halloween. In previous years it was possible to get halloween effects from years past by unboxing an old enough crate. These effects could come on any hat that came out of the crate you unboxed.

• Neutron Star
  
• Starstorm Slumber
  
• Starstorm Insomnia
  
• Tesla ♥♥♥♥

This year added the possible drop of an Unusualifier for taunts when unboxing the Creepy Crawly Case. This item is linked to a specific taunt but randomly applies an effect when consumed. This item has since stopped awarding halloween effects, and instead rewards Taunt Generation One effects.

• Hellish Inferno
  
• Spectral Swirl
  
• Infernal Flames
  
• Infernal Smoke

These taunts were unboxed from taunt crates, and was released on June 18th 2014. During the Halloween event these effects aren't unboxable, and instead the halloween effects will be unboxed.

• Showstopper
  
• Holy Grail
  
• Mega Strike
  
• Fountain of Delight
  
• Screaming Tiger
  
• 72
  
• Silver Cyclone
  
• Midnight Whirlwind

Restricted Crate Generation effects are only available from their associated crate, they cannot be unboxed from normal crates. Robotic Boogalo gave us our first RC Gen effects, and was released on May 17th 2013. The Robotic effects could come on any hat, not just hats that came out of this crate.

• Anti-Freeze
  
• Roboactive
  
• Green Black Hole
  
• Time Warp
  
• Phosphorous
  
• Sulphurous
  
• Electrostatic
  
• Power Surge
  
• Memory ♥♥♥♥
  
• Overclocked

The second set of RC Gen effects was released with the End of the Line update, on December 8th 2014 Patch. These effects only come from the End of the Line crate, and cannot be unboxed from normal crates; but could come on any hat, not just hats that came out of this crate.

• Death at Dusk
  
• Morning Glory
  
• Frostbite
  
• Molten Mallard

The Invasion community update added 9 RC Gen effects, and was released on October 6th 2015.

These effects only go on hats from these crates.

• Nebula
  
• Abduction
  
• Atomic
  
• Subatomic
  
• Electric Hat Protector
  
• Magnetic Hat Protector
  
• Voltaic Hat Protector
  
• Galactic Codex
  
• Ancient Codex

The Gargoyle update added 9 effects, and was released on October 6th 2015.

These effects only go on hats from these crates.

• Death by Disco
  
• It's a Mystery to Everyone
  
• It's a Puzzle to Me
  
• Ether Trail
  
• Nether Trail
  
• Ancient Eldritch
  
• Eldritch Flame

Added in the Gun Mettle Update on July 2 2015 and used for all skinned weapon unusual effects, these 4 effects show up on the barrel of the gun.

• Energy Orb
  
• Isotope
  
• Hot
  
• Cool