L33t Tr4nsl4t0r (Leet Translator)

Way too much code. Using arrays of characters, your switch constructs can be reduced to just a few lines of code, like so:

switch (c)
         {
           case 'a': sb.Append("4"); break;
           case 'e': sb.Append("3"); break;
           case 'i': sb.Append("1"); break;
           case 'o': sb.Append("0"); break;
           case 'A': sb.Append("4"); break;
           case 'E': sb.Append("3"); break;
           case 'I': sb.Append("1"); break;
           case 'O': sb.Append("0"); break;
           default: sb.Append(c); break;
         }

becomes:

string rep="43104310";
int index= "aeioAEIO".IndexOf(c);
if (index<0) sb.Append(c);
else sb.Append(rep[index]);

where rep is used to further improve readability.

:)