// e.g. TEST, TeST, TEsT, TESt, tEST, etc.
public static List<string> Permute(string s)
{
    List<string> listPermutations = new List<string>();

    char[] array = s.ToLower().ToCharArray();
    int iterations = (1 << array.Length) - 1;

    for (int i = 0; i <= iterations; i++)
    {
        for (int j = 0; j < array.Length; j++)
            array[j] = (i & (1 << j)) != 0
                          ? char.ToUpper(array[j])
                          : char.ToLower(array[j]);
        listPermutations.Add(new string(array));
    }
    return listPermutations;
}