I am grateful to Russell Kaye on the usenet group uk.education.teachers for this.

These are the 100 words most commonly misspelt by primary children:
(and a surprisingly large number of secondary, graduate and postgraduate students as well!)

Whoops!

1. there / their / there
(There is/over there/ their cat / they're going). You need to know the
difference.
2. two/to/too
(two fish, too many, to the seaside)
3. they
4. then
5. until
6. our
7. asked
8. off / of
9. through / threw
10. you're / your
11. clothes
12. looked
13. people
14. pretty
15. running
16. believe
17. little
18. things
19. him
20. because
21. thought
22. and
23. beautiful
24. it's / its (its bone / it's sunny)
25. went
26. where / were
27. stopped
28. very
29. morning
30. something
31. named
32. came
33. name
34. tried
35. here / hear
36. many
37. know
38. with
39. together
40. swimming
41. first
42. than
43. decided
44. friend
45. when
46. let's
47. mother
48. another
49. some
50. bought
51. getting
52. going
53. course
54. woman
55. animals
56. started
57. that's
58. would
59. again
60. heard
61. received
62. coming
63. said
64. wanted
65. bear
66. from
67. frightened
68. for / four
69. February
70. once
71. like
72. cousin
73. all right
74. happened
75. didn't
76. always
77. surprise
78. before
79. caught
80. every
81. different
82. sometimes
83. children
84. an (not a in front of a vowel e.g. an elephant)
85. school
86. jumped
87. around
88. dropped
89. babies
90. money
(There are 100 individual words!)