How to express an integer number in English words?

Mathematica 10 introduces IntegerName:

• IntegerName[n] gives a string containing the full English name of the integer n.

• IntegerName[n,"type"] gives a string of the specified type.

Possible types include:

• "DigitsWords" a combination of three-digit numbers and words
• "Words" using only words
• "Approximate" the first few digits plus thousands, millions, etc.
• "ApproximateWords" the first few digits as words plus thousands, etc.

Example:

IntegerName[#, "Words"] & /@
 {123456, 1000000001, 123456789123456789} // Column

"one hundred twenty-three thousand, four hundred fifty-six"

"one billion, one"

"one hundred twenty-three quadrillion, four hundred fifty-six trillion, 
 seven hundred eighty-nine billion, one hundred twenty-three million,
 four hundred fifty-six thousand, seven hundred eighty-nine"

Nested WolframAlpha approach, showing the intermediate steps:

numberString[a_, k_: 10] := 
 FixedPointList[
  StringReplace[#, 
    b : (DigitCharacter ..) :> 
     WolframAlpha["spell " <> b, {{"Result", 1}, "Plaintext"}]] &, a, 
  k]

numberString["123456"]

(*
==> {"123456", "123 thousand and 456", "one hundred twenty-three \
thousand and four hundred fifty-six", "one hundred twenty-three \
thousand and four hundred fifty-six"}
*)

numberString["123456789123456789123456789"]

(*
==> {"123456789123456789123456789", "123 septillion, 456 \
sextillion, 789 quintillion, 123 quadrillion, 456 trillion, 789 \
billion, 123 million, 456 thousand and 789", "one hundred \
twenty-three septillion, four hundred fifty-six sextillion, seven \
hundred eighty-nine quintillion, one hundred twenty-three \
quadrillion, four hundred fifty-six trillion, seven hundred \
eighty-nine billion, one hundred twenty-three million, four hundred \
fifty-six thousand and seven hundred eighty-nine", "one hundred \
twenty-three septillion, four hundred fifty-six sextillion, seven \
hundred eighty-nine quintillion, one hundred twenty-three \
quadrillion, four hundred fifty-six trillion, seven hundred \
eighty-nine billion, one hundred twenty-three million, four hundred \
fifty-six thousand and seven hundred eighty-nine"}
*)

Messy but a working method

inWords[n_] := Module[
   {r,
    numNames = {"", " one", " two", " three", " four", " five", 
      " six", " seven", " eight", " nine"},
    teenNames = {" ten", " eleven", " twelve", " thirteen", 
      " fourteen", " fifteen", " sixteen", " seventeen", " eighteen", 
      " nineteen"},
    tensNames = {"", " ten", " twenty", " thirty", " forty", " fifty",
       " sixty", " seventy", " eighty", " ninety"}, 
    decimals = {"", " thousand", " million", " billion", " trillion", 
  " quadrillion", " quintillion", " sextillion", " septillion", 
  " octillion", " nonillion", " decillion", " undecillion", 
  " duodecillion", " tredecillion", " quattuordecillion", 
  " quindecillion", " sexdecillion", " septendecillion", 
  " octodecillion", " Novemdecillion", " Vigintillion", " 66illion", 
  " 69illion"}

    }
   ,
   r = If[# != 0, 
        numNames[[# + 1]] <> " hundred" <> 
         If[#2 != 0 || #3 != 0, " and", ""], ""]
       <>
       Switch[#2
        , 0, numNames[[#3 + 1]]
        , 1, teenNames[[#3 + 1]]
        , _, tensNames[[#2 + 1]] <> numNames[[#3 + 1]]
        ]
      & @@@
     (PadLeft[FromDigits /@ Characters@StringReverse@#, 
         3] & /@ StringCases[StringReverse@IntegerString@n, 
        RegularExpression["\\d{1,3}"]]);

   StringJoin@
    Reverse@
     MapThread[
      If[# != "", StringJoin[##], ""] &, {r, Take[decimals, Length@r]}]
   ];

Output

In[279]:= FromDigits@Table[RandomInteger[9], {70}]
inWords@%

Out[279]= \
7317782180245641104170634561625559007936487060051082174021876603737988

Out[280]= " seven 69illion three hundred and seventeen 66illion seven \
hundred and eighty two Vigintillion one hundred and eighty \
Novemdecillion two hundred and forty five octodecillion six hundred \
and forty one septendecillion one hundred and four sexdecillion one \
hundred and seventy quindecillion six hundred and thirty four \
quattuordecillion five hundred and sixty one tredecillion six hundred \
and twenty five duodecillion five hundred and fifty nine undecillion \
seven decillion nine hundred and thirty six nonillion four hundred \
and eighty seven octillion sixty septillion fifty one sextillion \
eighty two quintillion one hundred and seventy four quadrillion \
twenty one trillion eight hundred and seventy six billion six hundred \
and three million seven hundred and thirty seven thousand nine \
hundred and eighty eight"