This one’s from Chris Meadowcroft:

const string EnglishListPrefix = "{";
const string EnglishListSuffix = "}";
const string IntermediateSeparator = ", ";
const string LastSeparator = " and ";
static string BuildStringWithEnglishListSyntax(IEnumerable<string> itemsEnumerable)
{
  StringBuilder result = new StringBuilder(EnglishListPrefix);
  using(IEnumerator<string> items = itemsEnumerable.GetEnumerator())
  {
    if(items.MoveNext())  // make sure it's not an empty list
    {
      bool isFirstString = true;
      bool isLastString = false;
      while(!isLastString)
      {
        string current = items.Current;
        isLastString = !items.MoveNext();
        if(!isFirstString)
        {
          result.Append(isLastString ? LastSeparator : IntermediateSeparator);
        }
        else
        {
          isFirstString = false;
        }
        result.Append(current);
       }
     }
   }
   result.Append(EnglishListSuffix);
   return result.ToString();
}