Simple text-to-binary converter written in C++
As already mentioned, your code is looking okay.  Here are my additions:


It's best not to use using namespace std.
Variables should be declared/initialized on their own line:

char letter = ' ';
char playAgain = 'y';

You don't need a newline between every line of code.  This will just make your program appear longer and harder to read.  Just separate lines into groups based on purpose.
Variable declarations/initializations should be placed as close in scope as possible to where they will be first used:

std::cout &lt;&lt; "Please enter a character, word, or phrase: ";
std::string word; // declaration is first used here
getline(std::cin, word, '\n');

Your main loop could use for (;;).  This is essentially an infinite loop, but it should do a break at the end if the user inputs a value other than 'Y':

for (;;)
{
    // do stuff

    std::cout &lt;&lt; "\n\nWould you like to try again? Y/N)";
    char playAgain;
    playAgain = std::toupper(playAgain);
    std::cin &gt;&gt; playAgain;
    std::cin.ignore();

    if (playAgain != 'Y')
    {
        break;
    }

    // playAgain is 'y', so do stuff again
}

Prefer to use std::string::size_type for the inner for-loop as it's the type returned from std::string::size().

Or if you have C++11, use a range-based for loop instead:

for (auto iter&amp; : word)
{
    letter = iter;

    // ...
}