This is by no means intended to be an exhaustive list. It simply lists misspelled words that I find particularly painful. The painful words are ones that are commonly used, and therefore theoretically easy to remember and naturally commonly misspelt. It is also an ongoing project and I add significant typos as I notice them. Please feel free to /msg me with words that could be appended to this list.

     Instructions for use (AA batteries not included): Commonly encountered misspelling of the word is followed by the correct version.The numbers below each entry refer to the google matches for each version of the word (misspelt version vs. correct version) and this section is intended to show that you are not the only culprit. When in doubt:

  1. Check the Battery status of your Guide
  2. Consult the guide.
  3. Replace misspelt word
  4. Click stumbit
  5. The Guide:

definately - definitely

The word definitely is related to the word finite. Recalling this should expel any doubts concerning the evil introduction of an 'a'.
351,000 vs. 3,460,000

humerous - humorous

The word humour/humor (depending on which side of the Atlantic you're on) is rarely misspelt so the infiltration of this particular horror baffles me no end.
16,500 vs. 684,000

occured - occurred

Also applies to Occurrence.
384,000 vs. 5,270,000

persistant - persistent

Another persistent yet easily avoidable error.
65,300 vs. 1,190,000

mispell - misspell

Duh. The word is a combination of mis and spell.
2,110 vs. 18,200

sentance - sentence

26,400 vs. 2,930,000

tendancy - tendency

30,500 vs. 1,310,000

usefull - useful

Applies to the entire list of words with the same ending such as thoughtful, helpful, plentiful and others.
26,400 vs. 2,930,000

existance - existence

158,000 vs. 4,360,000

diffrent - different

Applies to diffrence (difference). Commonly misspelt because of the way it's pronounced.
56,800 vs. 31,600,000

responsability - responsibility

Pity I can't provide statistics for the even more common responsable (responsible) but the word is correct in french and spanish.
56,600 vs. 3,090,000

commerical - commercial

(Thanks to baffo)
129,000 vs. 15,800,000

wierd - weird

A weird exception to the 'i' before 'e' except after 'c' rule.
129,000 vs. 15,800,000

speach - speech

Well, you write speak, so why not speech? Go figure.
46,700 vs. 6,250,000

See also : "A lot" is two words, commonly misspelled words, misspelt