Why?  Because they're idiot easy to break, and they're at least partly responsible for the fallout all over the rest of the Web from the Gawker break-in and account info leak a few weeks ago.

Source: The top 50 passwords you should never use - Graham Cluley, Naked Security, Sophos Labs, 2010-12-15.

I took the liberty of adding some commentary on certain passwords.

Truthfully, these are passwords that shouldn't have been used even before the Gawker break-in, let alone after.

1. 123456
2. password
3. 12345678
4. lifehack
   1. Believe it or not, this was a password used on Life Hack, one of the Gawker sites compromised.  Hang your head in shame.
5. qwerty
6. abc123
7. 111111
8. monkey
9. consumer
10. 12345
    1. The Spaceballs luggage combination wasn't #1 but was in the top 10??  Color me shocked!
11. 0
12. letmein
    1. Oh, the attacker got let in, all right.
13. trustno1
14. dragon
15. 1234567
16. baseball
17. superman
18. iloveyou
19. gizmodo
20. sunshine
21. 1234
    1. There we have every combination of serial numbers 4-7.  Pure genius.
22. princess
23. starwars
24. whatever
25. shadow
26. cheese
27. 123123
28. nintendo
29. football
30. computer
31. fuckyou
    1. You'd be surprised at the number of people who choose profane passwords.  They're certainly surprised when IT cracks them and it shows up in the list of weak passwords given to their bosses.  They're also surprised when they have to update their resumes after such an embarrassing disclosure.
    2. Profanity does not add to password complexity at all.  Yet another reason not to use it.
32. 654321
33. blahblah
34. passw0rd
    1. This trick is not clever.  It's old hat now, and the technique is as easily broken as pure dictionary words.
35. master
36. soccer
37. michael
38. 666666
39. jennifer
40. gawker
41. Password
    1. Capitalization used to help, but computers and network connections are fast enough now that it's as weak as all-lowercase dictionary words, for all intents and purposes.
42. jordan
43. pokemon
    1. Hackers catch 'em all~
44. michelle
45. killer
46. pepper
47. welcome
    1. Those who broke in certainly were.
48. batman
49. kotaku
50. internet