With performance metrics...

// 10,889,187 operations/sec
function capitalizeFirstLetter(string) {
    return string[0].toUpperCase() + string.slice(1);
}

// 10,875,535 operations/sec
function capitalizeFirstLetter(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
}

// 4,632,536 operations/sec
function capitalizeFirstLetter(string) {
    return string.replace(/^./, string[0].toUpperCase());
}

// 1,977,828 operations/sec
String.prototype.capitalizeFirstLetter = function() {
    return this.charAt(0).toUpperCase() + this.slice(1);
}