How can I make a .NET Windows Forms application that only runs in the System Tray?

The code project article Creating a Tasktray Application gives a very simple explanation and example of creating an application that only ever exists in the System Tray.

Basically change the Application.Run(new Form1()); line in Program.cs to instead start up a class that inherits from ApplicationContext, and have the constructor for that class initialize a NotifyIcon

static class Program
{
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);

        Application.Run(new MyCustomApplicationContext());
    }
}


public class MyCustomApplicationContext : ApplicationContext
{
    private NotifyIcon trayIcon;

    public MyCustomApplicationContext ()
    {
        // Initialize Tray Icon
        trayIcon = new NotifyIcon()
        {
            Icon = Resources.AppIcon,
            ContextMenu = new ContextMenu(new MenuItem[] {
                new MenuItem("Exit", Exit)
            }),
            Visible = true
        };
    }

    void Exit(object sender, EventArgs e)
    {
        // Hide tray icon, otherwise it will remain shown until user mouses over it
        trayIcon.Visible = false;

        Application.Exit();
    }
}

As mat1t says - you need to add a NotifyIcon to your application and then use something like the following code to set the tooltip and context menu:

this.notifyIcon.Text = "This is the tooltip";
this.notifyIcon.ContextMenu = new ContextMenu();
this.notifyIcon.ContextMenu.MenuItems.Add(new MenuItem("Option 1", new EventHandler(handler_method)));

This code shows the icon in the system tray only:

this.notifyIcon.Visible = true;  // Shows the notify icon in the system tray

The following will be needed if you have a form (for whatever reason):

this.ShowInTaskbar = false;  // Removes the application from the taskbar
Hide();

The right click to get the context menu is handled automatically, but if you want to do some action on a left click you'll need to add a Click handler:

    private void notifyIcon_Click(object sender, EventArgs e)
    {
        var eventArgs = e as MouseEventArgs;
        switch (eventArgs.Button)
        {
            // Left click to reactivate
            case MouseButtons.Left:
                // Do your stuff
                break;
        }
    }

I've wrote a traybar app with .NET 1.1 and I didn't need a form.
First of all, set the startup object of the project as a Sub Main, defined in a module.
Then create programmatically the components: the NotifyIcon and ContextMenu.
Be sure to include a MenuItem "Quit" or similar.
Bind the ContextMenu to the NotifyIcon.
Invoke Application.Run().
In the event handler for the Quit MenuItem be sure to call set NotifyIcon.Visible = False, then Application.Exit(). Add what you need to the ContextMenu and handle properly :)