Cool (Linux) applications have their own icon in the notification area. To add my own icon in the Gnome notification area using Mono only took a few lines of code. In the post below I show how you can add your icon and associate a tooltip and right click menu to it.

GTK provides the StatusIcon class to register your own icon in the notification area.

    StatusIcon  myStatusIcon;

You can provide and load your own icon, but for this example I used one of the GTK stock icons.

     myStatusIcon = StatusIcon.NewFromStock(Stock.Harddisk);  // I use a stock icon from Stock.*
                                                                                          // to avoid having to include an icon file
     myStatusIcon.Visible = true;                                             // Make sure we are displayed

Adding or changing a tooltip is as straighforward as assigning it:

      myStatusIcon.Tooltip = "Hello World ToolTip";                      // The message to show when the mouse hovers over the icon

To create a rightclick menu we need to register the PopupMenu event handler:

     myStatusIcon.PopupMenu += OnStatusIconPopupMenu;         // Link in the right click popup menu
     ...
     ...

    /* The Status Popup menu is shown when the user right clicks on the toolbar icon */

    protected void OnStatusIconPopupMenu(object sender, EventArgs e)
    {
        Menu popupMenu = new Menu();

        MenuItem helloItem = new MenuItem("About Hello World");
        helloItem.Show();
        helloItem.Activated += new EventHandler(OnHelloAboutActivated);

        popupMenu.Append(helloItem);
        popupMenu.Popup(null,null,null,3,Gtk.Global.CurrentEventTime);
    }

And that is really all there is to it. I have included the complete code below. If you create a new GTK solution in MonoDevelop you will end up with two files, Main.cs and MainWindows.Cs. Replace the MainWindow.CS file with the content below:

using System;
using Gtk;

public partial class MainWindow: Gtk.Window
{
	StatusIcon  myStatusIcon;
	AboutDialog aboutDialog;

	public MainWindow (): base (Gtk.WindowType.Toplevel)
	{

		/* The following lines create the notification area status icon */
		/* Make it visible and link in the right click popup menu       */ 

		myStatusIcon = StatusIcon.NewFromStock(Stock.Harddisk);  // I use a stock icon from Stock.*
		                                                         // to avoid having to include an icon file
		myStatusIcon.Tooltip = "Hello World ToolTip";            // The message to show when the mouse hovers over the icon
		myStatusIcon.Visible = true;                             // Make sure we are displayed
		myStatusIcon.PopupMenu += OnStatusIconPopupMenu;         // Link in the right click popup menu 

		/* Not necessary */
		Label AppWindowLabel = new Label("Hello World");
		this.Add(AppWindowLabel);

		Build ();
	}

	/* Called when the main application closes */

	protected void OnDeleteEvent (object sender, DeleteEventArgs a)
	{
		Application.Quit ();
		a.RetVal = true;
	}

	/* The Status Popup menu is shown when the user right clicks on the toolbar icon */

	protected void OnStatusIconPopupMenu(object sender, EventArgs e)
	{
		Menu popupMenu = new Menu();

		MenuItem helloItem = new MenuItem("About Hello World");
		helloItem.Show();
		helloItem.Activated += new EventHandler(OnHelloAboutActivated);

		popupMenu.Append(helloItem);
		popupMenu.Popup(null,null,null,3,Gtk.Global.CurrentEventTime);
	}

	/* If the user select the "About Hello World" menu option, we show the about dialog */

	protected void OnHelloAboutActivated(object sender, EventArgs e)
	{
		aboutDialog = new AboutDialog();

		aboutDialog.ProgramName = "The About Dialog Popup Demo";
		aboutDialog.Version = "1.0beta";
		aboutDialog.Comments = "The best things in life are simple!";
		aboutDialog.License = "Creative Commons";
		aboutDialog.Authors = new string[] { "Martijn Dijksterhuis" };
		aboutDialog.Website = "http://www.dijksterhuis.org";
		aboutDialog.Response += new ResponseHandler(OnHelloAboutClose);

		aboutDialog.Run();
	}

	/* Catch the "Close" and "X" button event from the about dialog box */

	protected void OnHelloAboutClose(object sender, ResponseArgs e)
	{
		if (e.ResponseId==ResponseType.Cancel || e.ResponseId==ResponseType.DeleteEvent)
		 aboutDialog.Destroy();
	}

}

This is a post from Martijn's C# Coding Blog.