They seem to have taken fairly reasonable precautions.  You can still reflect over the assemblies loaded in the AppDomain, so if you had time to poke through them, you might (or might not) find something in their custom assemblies that was interesting and exploitable, but it looks to me like the model is essentially what you'd get with a shared web host (plus the reasonable restriction of no file system or Internet access) and that model has been used to reasonably successfully isolate applications from each other and the host for many years now.  

As I mentioned in my comment on Tyler's answer, they've also implemented some controls to prevent applications from mounting DoS attacks on the service, such as limiting application runtime to 5 seconds, and application memory to 200KB.   

Security concerns with a "fiddle" site?
I don't know the reason fo that. Maybe you could help me

So code here creating a frame with 8 sliders.

public class MyFrame extends JFrame {


ImagePanel imagePanel;
final int Minimum = 0;
final int Maximum = 10;
final int NumberOfSpheres = 8;
final int NumberOfScales = 10;
MyRandomAccessFile file;

final String[] s = {"Друзья и Окружение",
        "Отношения", "Карьера и Бизнес",
        "Финансы", "Духовность и Творчество",
        "Личностный Рост", "Яркость Жизни", "Здоровье и Спорт"};

private final Color[] colors = {Color.RED, Color.ORANGE, Color.YELLOW, Color.GREEN, Color.BLUE, Color.PINK, Color.MAGENTA, Color.DARK_GRAY};

private final int[] array = {1, 0, 0, 0, 0, 0, 0, 0};

public MyFrame () {

    setTitle("Wheel Of Life");

    MySlider[] sliders = new MySlider[NumberOfSpheres];
    JButton saveButton = new JButton("Save");
    MyActionListener listener1 = new MyActionListener();
    saveButton.addActionListener(listener1);

    file = new MyRandomAccessFile();

    //String s3 = "0 0 0 0 0 0 0 0";
    //array = stringToIntArray(s3);
    array[1] = 4;

    JLabel[] labels = new JLabel[NumberOfSpheres];
    imagePanel = new ImagePanel(colors, array);
    System.out.println(array[1]);
    JPanel mainPanel = new JPanel();
    JPanel[] sliderPanels = new JPanel[NumberOfSpheres];
    JPanel mainSliderPanel = new JPanel(new GridLayout(4, 2, 20, 20));
    MyChangeListener listener = new MyChangeListener();

    for (int i = 0; i &lt; NumberOfSpheres; i++) {
        sliders[i] = new MySlider(s[i]);
        sliders[i].addChangeListener(listener);
        labels[i] = new JLabel(s[i]);
        labels[i].setForeground(colors[i]);
        labels[i].setFont(new Font("Droid Sans", Font.BOLD, 20));
        sliderPanels[i] = new JPanel();
        sliders[i].setMinimum(Minimum);
        sliders[i].setMaximum(Maximum);
        System.out.print(array[i]);
        sliders[i].setValue(4);
        sliders[i].setMajorTickSpacing(1);
        sliders[i].setMinorTickSpacing((int) 0.1);
        sliders[i].setPaintLabels(true);
        sliders[i].setPaintTicks(true);
        sliderPanels[i].setLayout(new GridLayout(2, 1, 5, 5));
        sliderPanels[i].add(sliders[i]);
        sliderPanels[i].add(labels[i]);
        mainSliderPanel.add(sliderPanels[i]);

    }

    mainPanel.setLayout(new BorderLayout());
    mainPanel.add(imagePanel, BorderLayout.CENTER);
    mainPanel.add(mainSliderPanel, BorderLayout.EAST);
    mainPanel.add(saveButton, BorderLayout.SOUTH);

    add(mainPanel);
}


And the output is this

4
99999999

How can final variable change its value at the execution time, what the hell???
Actual value of variable is depend on value that I writing at sliders[i].setValue(4); But I don't know how exactly...

And I tried to set a watchpoint for this variable... Doesn't working. One time its zero and next time program stop in this for loop value is 9, 4 or whatever...
