Hat Passwords:
In the Player Name Option Screen, name your Hominid the following to get the new head topping for two-player games:

cletus - unlock hat # 4
april - unlock hat # 5
superfly - unlock hat # 7
goodman - unlock hat # 8
grrl - unlock hat # 11
abe - unlock hat # 12
princess - unlock hat # 13
dandy - unlock hat # 14
behemoth - unlock hat # 24
tomfulp - unlock hat #2 (Brown Hair - male)

New Hats:
Blond Hair - Beat the final boss on Hard mode.
Pirate Hat - On level 1-1 destroy Hairy Nanny Daycare right after the miniboss.
Tom Fulp hair - Change your hominid's name to tomfulp
Jouster hat - Beat level 3-4 on Hard
Dimond tirera - Beat level 2-2 on Hard
Private Eye hat - Beat level 3-2 on Hard
Brown Hat - Kill at least 7 birds in level 2-5.
Area 51 Hat - Get to level 3-1
Curly Hair Wig - Blow up the Fish are like plants building in level 1-4
Soviet Hat - Get to level 2-1
Top Hat and Flower Ring - Beat level 1-1 with gore mode off on any difficulty
Shark Fin - Go to level 2-4 and dig underneath the snowmobile and suffocate

Unlockable Mini-Games:
Super Soviet Missle Mastar - Complete 1-5
Challenge - Complete World 1
PDA Games - (available at start)
Neutron Ball - beat 1-1, any difficulty
Piñata Boss - beat the first miniboss in 1-1, any difficulty

All Hats:
There are two ways to unlock a hat:
a) perform a task (e.g. blow up a certain building, finish a certain level)
b) use a special name as your name In most cases, you will hear an air horn sound when you unlock something . Here is a list of all hats in menu order and the tasks that unlock them:

30. Black and White hat - Beat 3-1

2. Brown Hair (male) - On hard, jump off the spaceship before the screen fades after beating the final boss

29. Goggles - In 1-4 beat the helicopter without losing tank (you still lose the take when the helicopter crashes down)

28. Ears - Beat 2-2

26. Yellow Shark Fin - In 2-4 dig/suffocate before riding snowmobile

27. Conical brown hat - Beat 2-2

24. Rooster Hat - In 1-4 destroy the Behemoth-logo building

25. Jester's Cap - Beat 3-4 on hard

23. Indian Feather - (available at start)

21. KGB Agent hat - Beat 2-1

22. Pirate Hat - In 1-1 destroy "Hairy Mommy Daycare"

20. Area 51 agent hat - Beat 3-2

18. Flower - (available at start)

19. Viking Hat - (available at start) 16.

Chef Hat - Beat 1-3

17. Conical purple hat - In 1-3 destroy "Castle of Crap" building

15. Nurse Cap In 3-5 - get to final boss (don't have to win)

14. Private Eye hat - Beat 3-2 on hard

13. Daisy-petal hat - Beat 2-3

12. Tiara Beat 2-2 on hard

11. Stove Pipe Hat - Beat 1-1 with gore off

9. Vietnamese Hat - (available at start)

10. Crown of Flowers - Beat 1-1 with gore off

8. Red Bandana - Beat 1-4

7. Jheri Curl - In 1-4 destroy "fish are like plants" building

6. Black Afro - Beat 1-5

5. Brown Wig/Pearls - Beat 1-3

4. Blond Wig/Pearls - Beat 1-2

3. Brown Hat- In 2-5, kill 7 birds

0. No Hat - (available at start)

1. Red Baseball Cap - (available at start)

All Hats, Levels unlocked:
To unlock all levels name your alien ROYGBIV.