Number letter counts
Mathematica, 38 bytes

StringLength@StringJoin@IntegerName[Range[1000]]


@Doorknob has a leaner implementation:

Tr@StringLength@IntegerName@Range@1000


(* 20961 *)



Explanation

IntegerName[Range[10]]


(* {"one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"} *)

StringJoin@IntegerName[Range[10]]


(* "onetwothreefourfivesixseveneightnineten" *)

StringLength@StringJoin@IntegerName[Range[10]]


(* 39 *)