The class demonstrates probably all possible string key codes in Unity that I've found inside UnityPlayer.dll binary. They can be used in Input.GetKey()

CompleteKeyCodesList.cs

public static class CompleteKeyCodesList
{
    public const int MaxJoysticksCount = 16;
    public const int MaxJoystickButtonsCount = 20;

    public readonly static string[] KeyboardCodes =
    {
        "backspace",
        "delete",
        "tab",
        "clear",
        "return",
        "pause",
        "escape",
        "space",

        "[0]",
        "[1]",
        "[2]",
        "[3]",
        "[4]",
        "[5]",
        "[6]",
        "[7]",
        "[8]",
        "[9]",
        "[.]",
        "[/]",
        "[*]",
        "[-]",
        "[+]",
        "equals",
        "enter",

        "up",
        "down",
        "right",
        "left",

        "insert",
        "home",
        "end",
        "page up",
        "page down",

        "f1",
        "f2",
        "f3",
        "f4",
        "f5",
        "f6",
        "f7",
        "f8",
        "f9",
        "f10",
        "f11",
        "f12",
        "f13",
        "f14",
        "f15",

        "0",
        "1",
        "2",
        "3",
        "4",
        "5",
        "6",
        "7",
        "8",
        "9",
        "-",
        "=",

        "!",
        "@",
        "#",
        "$",
        "%",
        "^",
        "&",
        "*",
        "(",
        ")",
        "_",
        "+",

        "[",
        "]",
        "`",
        "{",
        "}",
        "~",

        ";",
        "'",
        "\\",
        ":",
        "\"",
        "|",

        ",",
        ".",
        "/",
        "<",
        ">",
        "?",

        "a",
        "b",
        "c",
        "d",
        "e",
        "f",
        "g",
        "h",
        "i",
        "j",
        "k",
        "l",
        "m",
        "n",
        "o",
        "p",
        "q",
        "r",
        "s",
        "t",
        "u",
        "v",
        "w",
        "x",
        "y",
        "z",

        "numlock",
        "caps lock",
        "scroll lock",
        "right shift",
        "left shift",
        "right ctrl",
        "left ctrl",
        "right alt",
        "left alt",
        "right cmd",
        "left cmd",
        "right super",
        "left super",
        "alt gr",

        "compose",
        "help",
        "print screen",
        "sys req",
        "break",
        "menu",
        "power",
        "euro",
        "undo",
    };

    public readonly static string[] MouseCodes =
    {
        "mouse 0",
        "mouse 1",
        "mouse 2",
        "mouse 3",
        "mouse 4",
        "mouse 5",
        "mouse 6",
    };

    // joystick button 0, joystick button 1, ..., joystick button 19
    public readonly static string[] AnyJoystickCodes;

    // joystick 1 button 0, joystick 1 button 1, ..., joystick 16 button 19
    public readonly static string[][] JoystickCodes;

    public readonly static string[] AllCodes;

    static CompleteKeyCodesList ()
    {
        int i, allCodesIdx = 0;

        AllCodes = new string[KeyboardCodes.Length + MouseCodes.Length + ( MaxJoysticksCount + 1 ) * MaxJoystickButtonsCount ];

        for( i = 0; i < KeyboardCodes.Length; i++, allCodesIdx++ )
            AllCodes[allCodesIdx] = KeyboardCodes[i];

        for( i = 0; i < MouseCodes.Length; i++, allCodesIdx++ )
            AllCodes[allCodesIdx] = MouseCodes[i];

        AnyJoystickCodes = new string[MaxJoystickButtonsCount];
        for( i = 0; i < MaxJoystickButtonsCount; i++, allCodesIdx++ )
        {
            AnyJoystickCodes[i] = "joystick button " + i;
            AllCodes[allCodesIdx] = AnyJoystickCodes[i];
        }

        JoystickCodes = new string[MaxJoysticksCount][];
        for( i = 0; i < MaxJoysticksCount; i++ )
        {
            JoystickCodes[i] = new string[MaxJoystickButtonsCount];
            for( int j = 0; j < MaxJoystickButtonsCount; j++, allCodesIdx++ )
            {
                JoystickCodes[i][j] = "joystick " + ( i + 1 ) + " button " + j;
                AllCodes[allCodesIdx] = JoystickCodes[i][j];
            }
        }
    }
}